Nitride semiconductor ultraviolet light-emitting element

ABSTRACT

A nitride semiconductor ultraviolet light-emitting element is provided. The element includes a light-emitting element structure part with an n-type layer, an active layer, and a p-type layer stacked vertically, which are made of AlGaN-based semiconductors with wurtzite structure. The n-type layer has an n-type AlGaN-based semiconductor, the active layer has well layers including an AlGaN based semiconductor, and the p-type layer has a p-type AlGaN-based semiconductor. Each semiconductor layer in the n-type and the active layers is an epitaxially grown layer having a surface on which multi-step terraces parallel to the (0001) plane are formed. The n-type layer has first Ga-rich regions which include n-type AlGaN regions in which an AlGaN composition ratio is an integer ratio of Al 1 Ga 1 N 2 . The well layer includes a second Ga-rich region, which includes an AlGaN region in which an AlGaN composition ratio is an integer ratio of Al 1 Ga 2 N 3 .

TECHNICAL FIELD

The present invention relates to a nitride semiconductor ultravioletlight-emitting element having a peak emission wavelength within a rangeof 300 nm to 327 nm and comprising a light-emitting element structurepart with an n-type layer, an active layer, and a p-type layer stackedvertically which are made of AlGaN-based semiconductors with wurtzitestructure.

BACKGROUND ART

In general, there are a lot of nitride semiconductor light-emittingelements with a light-emitting element structure comprising a pluralityof nitride semiconductor layers formed by epitaxial growth on asubstrate such as sapphire. A nitride semiconductor layer is representedby the general formula: Al_(1-x-y)Ga_(x)In_(y)N (0≤x≤1, 0≤y≤1, 0≤x+y≤1).

The light-emitting element structure of a light-emitting diode has adouble hetero structure in which an active layer made of a nitridesemiconductor layer having a single-quantum-well structure (SQW) or amulti-quantum-well structure (MQW) is sandwiched between two claddinglayers of an n-type nitride semiconductor layer and a p-type nitridesemiconductor layer. When the active layer is an AlGaN-basedsemiconductor, by adjusting an AlN mole fraction (also referred to as anAl composition ratio), band gap energy can be adjusted within a rangewhere band gap energies that can be taken by GaN and AlN (about 3.4 eVand about 6.2 eV) are lower and upper limits, so that an ultravioletlight-emitting element having a light emission wavelength of about 200nm to about 365 nm is obtained. Specifically by passing a forwardcurrent from the p-type nitride semiconductor layer toward the n-typenitride semiconductor layer, light emission corresponding to the bandgap energy due to recombination of carriers (electrons and holes) occursin the active layer. In order to supply the forward current from theoutside, a p-electrode is provided on the p-type nitride semiconductorlayer, and an n-electrode is provided on the n-type nitridesemiconductor layer.

When the active layer is an AlGaN-based semiconductor, the n-typenitride semiconductor layer and the p-type nitride semiconductor layersandwiching the active layer are composed of the AlGaN-basedsemiconductor having a higher AlN mole fraction than the active layer.However, since the p-type nitride semiconductor layer of a high AlN molefraction is difficult to form a good ohmic contact with the p-electrode,it is generally performed to form a p-type contact layer made of ap-type AlGaN semiconductor with a low AlN mole fraction (specificallyp-GaN), which can have a good ohmic contact with the p-electrode, to theuppermost layer of the p-type nitride semiconductor layer. Since the AlNmole fraction of the p-type contact layer is smaller than that of theAlGaN-based semiconductor constituting the active layer, ultravioletlight emitted toward the p-type nitride semiconductor layer side fromthe active layer is absorbed in the p-type contact layer and cannot beeffectively extracted to the outside of the element. Therefore, atypical ultraviolet light-emitting diode having an active layer made ofan AlGaN-based semiconductor employs an element structure asschematically shown in FIG. 13 . The ultraviolet light emitted towardthe n-type nitride semiconductor layer side from the active layer iseffectively extracted to the outside of the element (e.g., see PatentDocuments 1 and 2, Non-Patent Documents 1 and 2, etc. listed below).

As shown in FIG. 13 , the typical UV light-emitting diode is constructedby depositing an n-type AlGaN-based semiconductor layer 103, an activelayer 104, a p-type AlGaN-based semiconductor layer 105, and a p-typecontact layer 106 on a template 102 formed by depositing an AlGaN-basedsemiconductor layer 101 (e.g., AlN layer) on a substrate 100 such as asapphire substrate, and etching away respective portions of the activelayer 104, the p-type AlGaN-based semiconductor layer 105, and thep-type contact layer 106 until the n-type AlGaN-based semiconductorlayer 103 is exposed, and forming an n-electrode 107 on the exposedsurface of the n-type AlGaN-based semiconductor layer 103 and anp-electrode 108 on the surface of the p-type contact layer 106.

In addition, in order to improve luminous efficiency (internal quantumefficiency) by carrier recombination in the active layer, the activelayer is formed in a multi-quantum-well structure, and an electronblocking layer is provided on the active layer.

On the other hand, it has been reported that in a cladding layer made ofan n-type AlGaN-based semiconductor layer, compositional modulation dueto segregation of Ga occurs, and stratiform regions with a locally lowerAlN mole fraction are formed extending obliquely to the surface of thecladding layer (for example, see Patent Document 3, Non-Patent Documents1, 2, etc. described below). Because the band gap energy of theAlGaN-based semiconductor layer with the locally lower AlN mole fractionis also locally reduced, it is reported in Patent Document 3 that thecarriers in the cladding layer is easily localized in the stratiformregions, which can provide low resistance current paths to the activelayer, and that the luminous efficiency of the ultravioletlight-emitting diode can be improved.

PRIOR ART DOCUMENTS Patent Document

-   Patent Document 1: WO2014/178288-   Patent Document 2: WO2016/157518-   Patent Document 3: WO2019/159265

Non-Patent Document

-   NON-PATENT DOCUMENT 1: Y. Nagasawa, et al., “Comparison of    Al_(x)Ga_(1-x)N multiple quantum wells designed for 265 and 285 nm    deep-ultraviolet LEDs grown on AlN templates having macrosteps”,    Applied Physics Express 12, 064009 (2019)-   NON-PATENT DOCUMENT 2: K. Kojima, et al., “Carrier localization    structure combined with current micropaths in AlGaN quantum wells    grown on an AlN template with macrosteps”, Applied Physics letter    114, 011102 (2019)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An ultraviolet light-emitting element composed of the AlGaN-basedsemiconductors is produced on a substrate such as a sapphire substrateby a well-known epitaxial growth method such as, for example, ametalorganic vapor phase epitaxy (MOVPE) method. However, when producingthe ultraviolet light-emitting element, the characteristics of theultraviolet light-emitting element (properties such as emissionwavelength, wall plug efficiency, forward bias, etc.) fluctuate underthe influence by drift of the crystal growth apparatus, so that theproducing at a stable yield is not always easy.

The drift of the crystal growth apparatus occurs due to a change in theeffective temperature of the crystal growth site or the like owing todeposits on trays, chamber walls, or the like. Therefore, in order tosuppress the drift, conventionally treatments such as fine adjustmentsof the set temperature and the composition of source gases byexperienced persons with examining the growth history fixing the growthschedule for a certain period, and performing maintenance such ascleaning in the same manner for a certain period are performed, but itis difficult to eliminate the drift completely.

The present invention has been made in view of the above-mentionedproblems, and an object of the present invention is to provide a nitridesemiconductor ultraviolet light-emitting element which can be stablyproduced with variation in characteristics due to the drift of thecrystal growth apparatus or the like being suppressed.

Means for Solving the Problem

The present invention, in order to achieve the above object, provides anitride semiconductor ultraviolet light-emitting element having a peakemission wavelength within a range of 300 nm to 327 nm and comprising alight-emitting element structure part in which an n-type layer, anactive layer, and a p-type layer made of an AlGaN-based semiconductor ofwurtzite structure are stacked vertically, wherein

the n-type layer is composed of an n-type AlGaN-based semiconductor,

the active layer disposed between the n-type layer and the p-type layerhas a quantum-well structure having one or more well layers composed ofa GaN-based semiconductor,

the p-type layer is composed of a p-type AlGaN-based semiconductor,

each semiconductor layer in the n-type layer and the active layer is anepitaxially grown layer having a surface on which multi-step terracesparallel to a (0001) plane are formed,

the n-type layer has a plurality of first Ga-rich regions, the pluralityof first Ga-rich regions being stratiform regions uniformly distributedin the n-type layer with locally lower AlN mole fraction and includingn-type AlGaN regions in which an AlGaN composition ratio is an integerratio of Al₁Ga₁N₂,

each extending direction of the first Ga-rich regions on a first planeperpendicular to an upper surface of the n-type layer is inclined withrespect to an intersection line between the upper surface of the n-typelayer and the first plane,

an AlN mole fraction of an n-type body region outside the stratiformregions in the n-type layer is within a range of 54% to 66%.

Furthermore, the present invention, in order to achieve the aboveobject, provides a manufacturing method of a nitride semiconductorultraviolet light-emitting element having a peak emission wavelengthwithin a range of 300 nm to 327 nm and comprising a light-emittingelement structure part in which an n-type layer, an active layer, and ap-type layer made of an AlGaN-based semiconductor of wurtzite structureare stacked vertically the method comprising

a first step of epitaxially growing the n-type layer of an n-typeAlGaN-based semiconductor on an underlying part including a sapphiresubstrate having a main surface inclined by a predetermined angle withrespect to a (0001) plane, and making multi-step terraces parallel tothe (0001) plane appear on a surface of the n-type layer,

a second step of epitaxially growing the active layer of a quantum-wellstructure having one or more well layers composed of a GaN-basedsemiconductor on the n-type layer, and making multi-step terracesparallel to the (0001) plane appear on a surface of the well layer, and

a third step of forming the p-type layer of a p-type AlGaN-basedsemiconductor on the active layer by epitaxial growth, wherein

in the first step, a target AlN mole fraction of the n-type layer is setwithin a range of 54% to 66% and a plurality of first Ga-rich regionsare grown so as to extend obliquely upward, the plurality of firstGa-rich regions being stratiform regions uniformly distributed in then-type layer with locally lower AlN mole fraction and including n-typeAlGaN regions in which an AlGaN composition ratio is an integer ratio ofAl₁Ga₁N₂.

The AlGaN-based semiconductor is represented by the general formulaAl_(1-x)Ga_(x)N (0≤x≤1), but the semiconductor may contain a traceamount of an impurity such as a Group 3 element such as B or In or aGroup 5 element such as P, as far as the band gap energy is within thelower limit and the upper limit of the band gap energy that can beobtained by GaN and AlN, respectively. The GaN-based semiconductor isbasically a nitride semiconductor composed of Ga and N, and may containa trace amount of an impurity such as a Group 3 element such as Al, B,or In or a Group 5 element such as P. The AlN-based semiconductor isbasically a nitride semiconductor composed of Al and N, and may containa trace amount of an impurity such as a Group 3 element such as Ga, B,or In or a Group 5 element such as P. Therefore, in the presentapplication, the GaN-based semiconductor and the AlN-based semiconductorare a part of the AlGaN-based semiconductor, respectively.

Furthermore, the n-type or p-type AlGaN-based semiconductor is anAlGaN-based semiconductor in which Si or Mg or the like is doped as adonor impurity or an acceptor impurity. In the present application, theAlGaN-based semiconductor, not specified as p-type or n-type, meansundoped AlGaN-based semiconductor, but even if undoped, a trace amountof donor or acceptor impurities to the extent of being inevitably mixedmay be included. The first plane is not an exposed surface or a boundarysurface between the n-type layer and other semiconductor layer which isspecifically formed in the manufacturing process of the n-type layer butis a virtual plane extending in parallel to the vertical direction inthe n-type layer. Furthermore, in this specification, an AlGaN-basedsemiconductor layer, a GaN-based semiconductor layer, and an AlN-basedsemiconductor layer are semiconductor layers composed of the AlGaN-basedsemiconductor, the GaN-based semiconductor, and the AlN-basedsemiconductor, respectively.

According to the nitride semiconductor ultraviolet light-emittingelement having the above-mentioned features or the manufacturing methodof the nitride semiconductor ultraviolet light-emitting element havingthe above-mentioned features, it is expected that a nitridesemiconductor ultraviolet light-emitting element having a peak emissionwavelength within a range of 300 nm to 327 nm is stably produced byprimarily utilizing metastable AlGaNs formed in the first Ga-rich regionin the n-type layer as described later and secondly utilizing the welllayer composed of a GaN-based semiconductor to suppress thecharacteristics variation caused by the drift of the crystal growthapparatus or the like.

First, a “metastable AlGaN” in which an AlGaN composition ratio isrepresented by a predetermined integer ratio will be described.

Usually a ternary mixed crystal such as AlGaN is a crystalline state inwhich group 3 elements (Al and Ga) are randomly mixed and isapproximately described by “random non-uniformity”. However, because thecovalent bond radiuses of Al and Ga are different, the higher thesymmetry of the atomic arrangement of Al and Ga in the crystalstructure, the more stable the structure is in general.

The AlGaN-based semiconductors with wurtzite structure can have twotypes of arrangements, a random arrangement without symmetry and astable symmetric arrangement. Here, a state in which the symmetricarrangement is dominant appears at a constant rate. In the “metastableAlGaN” described later, in which an AlGaN composition ratio (compositionratio of Al, Ga, and N) is represented by a predetermined integer ratio,a periodic symmetric arrangement structure of Al and Ga develops.

In the periodic symmetric arrangement structure, even if amount of Gasupplied to the crystal growth surface is slightly increased ordecreased, the mixed crystal mole fraction becomes slightly stable interms of energy because of the high symmetry and it is possible toprevent proliferation of places where amount of easily mass-transferringGa is extremely increased. That is, by utilizing the properties of the“metastable AlGaN” formed in the first Ga-rich region in the n-typelayer, as an AlGaN-based semiconductor, even if the variation of themixed crystal mole fraction due to the drift of the crystal growthapparatus or the like occurs, the variation of the mixed crystal molefraction in the first Ga-rich regions which provide the low resistancecurrent paths to the active layer as described later is locallysuppressed. Consequently stable carrier supply from the n-type layerinto the active layer can be realized and variation in devicecharacteristics can be suppressed, so that it is expected to stablyproduce a nitride semiconductor ultraviolet light-emitting element thatachieves the desired characteristics.

Next, the AlGaN composition ratio in which Al and Ga can be in theperiodic symmetric arrangement in the (0001) plane will be described.

FIG. 1 shows a schematic diagram of one unit cell (two monolayers) alongthe c-axis of AlGaN. In FIG. 1 , open circles indicate sites where atomsof Group 3 elements (Al, Ga) are located, and solid circles indicatesites where atoms of Group 5 elements (N) are located.

The site planes (A3 plane, B3 plane) of the Group 3 elements and thesite planes (A5 plane, B5 plane) of the Group 5 element shown byhexagons in FIG. 1 are both parallel to the (0001) plane. Six sites ateach vertex of the hexagon and one site at the center of the hexagon arepresent at each site of the A3 and A5 planes (collectively plane A). Thesame applies to the B3 plane and the B5 plane (collectively B plane),but in FIG. 1 , it illustrates only three sites present in the hexagonof the B plane. Each site of the A plane is overlapped in the c-axisdirection, each site of the B plane is overlapped in the c-axisdirection. However, the atom (N) of one site on the B5 plane forms aquaternary bond with the atoms (Al, Ga) of the three sites on the A3plane located above the B5 plane and the atom (Al, Ga) of one site onthe B3 plane located below the B5 plane, and the atom (Al, Ga) of onesite on the B3 plane forms a quaternary bond with the atom (N) of onesite on the B5 plane located above the B3 plane and the atoms (N) ofthree sites on the A5 plane located below the B3 plane, so that eachsite on the A plane does not overlap each site on the B plane in thec-axis direction, as shown in FIG. 1 .

FIG. 2 shows a positional relationship between each site of the A planeand the B plane, as a plan view as viewed from the c-axis direction. Inboth the A and B planes, each of the six vertices of the hexagon isshared by the other two hexagons adjacent to each other, and the site atthe center is not shared with the other hexagons, so there aresubstantially three atomic sites within one hexagon. Thus, there are sixsites of Group 3 element atoms (Al, Ga) and six sites of Group 5 elementatoms (N) per unit cell. Therefore, the following five cases exist asAlGaN composition ratios expressed by the integer ratio excluding GaNand AlN.

1) Al₁Ga₅N₆

2) Al₂Ga₄N₆(═Al₁Ga₂N₃)

3) Al₃Ga₃N((═Al₁Ga₁N₂)

4) Al₄Ga₂N((═Al₂Ga₁N₃)

5) Al₅Ga₁N₆

FIG. 3 schematically shows the A3 plane and the B3 plane of the group 3element in the above five combinations. Ga is indicated by a solidcircle, and Al is indicated by an open circle.

In the case of Al₁Ga₅N₆ shown in FIG. 3 (A), Ga is located at six vertexsites of the A3 plane and six vertex sites and one center site of the B3plane, and Al is located at one center site of the A3 plane.

In the case of Al₁Ga₂N₃ shown in FIG. 3 (B), Ga is located at threevertex sites and one center site of the A3 and B3 planes, and Al islocated at three vertex sites of the A3 and B3 planes.

In the case of Al₁Ga₁N₂ shown in FIG. 3 (C), Ga is located at threevertex sites and one center site of the A3 plane and three vertex sitesof the B3 plane, and Al is located at three vertex sites of the A3 planeand three vertex sites and one center site of the B3 plane.

In the case of Al₂Ga₁N₃ shown in FIG. 3 (D), Ga is located at threevertex sites of the A3 and B3 planes, Al is located at three vertexsites and one center site of the A3 and B3 planes. This is equivalent toswapping the positions of Al and Ga in Al₁Ga₂N₃ shown in FIG. 3 (B).

In the case of Al₅Ga₁N₆ shown in FIG. 3 (E), Ga is located at one centersite of the A3 plane, Al is located at six vertex sites of the A3 planeand six vertex sites and one center site of the B3 plane. This isequivalent to swapping the positions of Al and Ga in Al₁Ga₅N₆ shown inFIG. 3 (A).

In each of FIGS. 3(A)-3(E), assuming another hexagon whose center hasmoved to any one of the six vertices of the hexagon, it can be seen thatAl or Ga located at the six vertex sites of the A3 plane is equivalentto Al or Ga located at the three vertex sites and the one center site ofthe A3 plane, and Al or Ga located at the one center of the A3 plane isequivalent to Al or Ga located at the three vertex sites of the A3plane. The same applies to the B3 plane. In addition, in each of FIGS.3(A), 3(C) and 3(E), A3 and B3 planes may be replaced with each other.

In each of FIGS. 3(A)-3(E), in both A3 and B3 planes, the atomicarrangement of Al and Ga is maintained in symmetry. Further, even if thecenter of the hexagon is moved, the atomic arrangement of Al and Ga ismaintained in symmetry.

Furthermore, in the A3 and B3 planes of FIGS. 3(A)-3(E), when thehexagonal site plane is arranged repeatedly in a honeycomb shape,looking at each site in a direction parallel to the (0001) plane, forexample, in [11-20] direction or [10-10] direction, the state in whichAl and Ga is located periodically repeatedly or either Al or Ga islocated continuously appears. Therefore, it can be seen that an atomicarrangement will be the periodic and symmetric atomic arrangement in therespective cases.

Hereinafter, Al_(x1)Ga_(1-x1)N of the AlN mole fraction x1 (x1=⅙, ⅓, ½,⅔, ⅚) corresponding to AlGaN composition ratios of above-mentioned 1) to5) is referred to as the “first metastable AlGaN”, for convenience ofexplanation. In the first metastable AlGaN, the atomic arrangement of Aland Ga becomes a periodic and symmetric arrangement, resulting in anenergetically stable AlGaN.

Next, when the site plane indicated by the hexagon shown in FIG. 1 isextended to 2 unit cells (4 monolayers), there are two planes for eachof the site planes of the Group 3 element (A3 plane, B3 plane) and twoplanes for each of the site planes of the Group 5 element (A5 plane, B5plane), respectively and there are 12 sites for atoms of the Group 3element (Al, Ga) and 12 sites for atoms of the Group 5 element (N) per 2unit cells. Therefore, as AlGaN composition ratios expressed by theinteger ratio excluding GaN and AlN, the following six combinationsexist in addition to the AlGaN composition ratios of above-mentioned 1)to 5).

6) Al₁Ga₁₁N₁₂ (=GaN+Al₁Ga₅N₆)

7) Al₃Ga₉N₁₂ (═Al₁Ga₃N₄=Al₁Ga₅N₆+Al₁Ga₂N₃)

8) Al₅Ga₇N₁₂ (=Al₁Ga₂N₃+Al₁Ga₁N₂)

9) Al₇Ga₅N₁₂ (=Al₁Ga₁N₂+Al₂Ga₁N₃)

10) Al₉Ga₃N₁₂ (═Al₃Ga₁N₄=Al₂Ga₁N₃+Al₅Ga₁N₆)

11) Al₁₁Ga₁N₁₂ (=Al₅Ga₁N₆+AlN)

However, since these six AlGaN composition ratios of 6) to 11) arecombinations of two AlGaN composition ratios among the first metastableAlGaN, GaN and AlN, located before and after them, the c-axis symmetryis likely to be disturbed, and the stability is lowered than the firstmetastable AlGaN. But these six AlGaN are more stable than AlGaN in arandom asymmetric arrangement state because the symmetry of the atomicarrangement of Al and Ga in the A3 and B3 planes is the same as that inthe first metastable AlGaN. Hereinafter, Al_(x2)Ga_(1-x2)N of the AlNmole fraction x2 (x2= 1/12, ¼, 5/12, 7/12, ¾, 11/12) corresponding toAlGaN composition ratios of above-mentioned 6) to 11) is referred to asthe “second metastable AlGaN”, for convenience of explanation. Asdescribed above, the first and second metastable AlGaN have a stablestructure due to the symmetry of the atomic arrangement of Al and Ga inthe crystalline structure. Hereinafter, the first and second metastableAlGaN are collectively referred to as “metastable AlGaN”.

To grow AlGaN with maintaining constant crystal quality it is requiredto perform crystal growth at a high temperature of 1000° C. or higher.However, it is assumed that Ga moves around at 1000° C. or higher evenafter atoms reach the sites of the crystal surface. On the other hand,since Al tends to adsorb to the surface unlike Ga, the movement afterentering the site is strongly restricted though it is considered to movesomewhat.

Therefore, even though Al₁Ga₅N(of above-mentioned 1), Al₁Ga₁₁N₁₂ ofabove-mentioned 6), and Al₁Ga₃N₄ of above-mentioned 7) are themetastable AlGaN, since AlN mole fractions are all less than or equal to25% and Ga composition ratios are high, Ga movement is intense at agrowth temperature around 1000° C., the symmetry of atomic arrangementis disturbed, and the atomic arrangement of Al and Ga is close to randomcondition. As a result, it is considered that the stability describedabove is reduced compared with other metastable AlGaN.

Next, the “first Ga-rich region” will be described. In the nitridesemiconductor ultraviolet light-emitting element and the manufacturingmethod of the nitride semiconductor ultraviolet light-emitting elementwith the above-described features, since each semiconductor layer in then-type layer and the active layer is an epitaxially grown layer having asurface on which multi-step terraces parallel to the (0001) plane areformed, in the n-type layer, the easily mass-transferring Ga moves onthe terrace region and concentrates on a boundary region betweenadjacent terraces, thereby forming a region having a lower AlN molefraction than the terrace region. This boundary region extends obliquelyupward with respect to the (0001) plane along with the epitaxial growthof n-type AlGaN layer of the n-type layer, so that stratiform regionswith locally lower AlN mole fraction are formed with uniformly dispersedin the n-type layer. Here, if the amount of Ga mass-transferring issufficiently large, the stratiform region may be the first Ga-richregion which includes the n-type AlGaN region of the metastable AlGaN,in which the AlGaN composition ratio is Al₁Ga₁N₂.

Specifically, since the AlN mole fraction of the n-type body region inthe n-type layer (approximated by the target AlN mole fraction of then-type layer) is in the range of 54% to 66%, the nearest metastableAlGaN that can be included in the stratiform regions formed with themass-transfer of Ga from the n-type body region becomes the metastableAlGaN with the AlGaN composition ratio of Al₁Ga₁N₂, and the differencebetween the AlN mole fractions of the n-type body region and thestratiform region is 5% or more. Therefore, the amount of Gamass-transferring is sufficiently large in the most of the stratiformregion, and the first Ga-rich region which includes the n-type AlGaNregion of the metastable AlGaN with the AlGaN composition ratio ofAl₁Ga₁N₂ is stably formed. Namely, the first Ga-rich regions aredominantly present in the stratiform regions, and uniformly distributedin the n-type layer as a result. However, since the crystal growth ofAlGaN is a random process so that the amount of Ga mass-transferringvaries randomly a region which does not include the n-type AlGaN regionof the metastable AlGaN with the AlGaN composition ratio of Al₁Ga₁N₂,that is, a region having a locally lower AlN mole fraction which islarger than 50%, can be formed.

The metastable AlGaN, in which the AlGaN composition ratio is Al₁Ga₁N₂,is present within the first Ga-rich region, so that variation in amountof Ga supplied into the first Ga-rich region are absorbed in themetastable AlGaN. That is, in the first Ga-rich region, when the Gasupply amount increases, the metastable AlGaN increases, and when the Gasupply amount decreases, the metastable AlGaN decreases, and as aresult, the variation of the AlN mole fraction in the first Ga-richregion is suppressed. Therefore, in the first Ga-rich region, thevariation of the Ga supply amount due to the drift of the crystal growthapparatus or the like is absorbed, and the metastable AlGaN in which theAlGaN composition ratio is Al₁Ga₁N₂(AlN mole fraction is 50%) is stablyformed. That is, the variation of the AlN mole fraction in the firstGa-rich region is suppressed against the variation of the Ga supplyamount.

However, as described above, since a state of the random asymmetricarrangement and a state of the regular symmetric arrangement can usuallycoexist in the crystal growth of AlGaN, a region of the metastable AlGaNin the state of the regular symmetric arrangement, in which the AlN molefraction is 50%, is stably formed in the first Ga-rich region, and aregion in which the AlN mole fraction fluctuates slightly (for example,about 0 to 3%) from 50% is also mixed therein. Therefore, the AlN molefraction in the first Ga-rich region is concentrated and distributednear the AlN-mole fraction (50%) of the metastable AlGaN in which theAlGaN composition ratio is Al₁Ga₁N₂.

In addition, since the AlN mole fraction of the n-type body region is inthe range of 54% to 66%, the difference between the AlN mole fractionsof the first Ga-rich region and the n-type body region is stably ensuredto be 4% or more. Thus, the carriers in the n-type layer are localizedin the first Ga-rich region having a lower band gap energy than then-type body region, and a current can preferentially flow through thefirst Ga-rich region stably in the n-type layer, thereby suppressingvariation in characteristics of the nitride semiconductor ultravioletlight-emitting element. As described above, since a region which doesnot reach the first Ga-rich region and has a locally lower AlN molefraction which is larger than 50% can be formed, if the differencebetween the AlN mole fractions of this region and the n-type body regionis 4% or more, the carriers in the n-type layer are also localized inthis region and a current can flow through this region as well as thefirst Ga-rich region.

Next, the “well layer” will be described. Since each semiconductor layerin the n-type layer and the active layer is an epitaxially grown layerhaving a surface on which multi-step terraces parallel to the (0001)plane are formed, a boundary region between adjacent terraces of themulti-step terraces of the well layer is an inclined region inclinedwith respect to the (0001) plane connecting between the adjacentterraces (see Non-Patent Documents 1 and 2 above). Incidentally theinclined region consists of many steps (steps of one unit cell) andmacrosteps (steps of multiple unit cells), and the (0001) surface shownstepwise on the inclined region is distinguished from the terracesurface of the multi-step terraces.

With growing of the side surfaces of the terrace edges toward lateraldirection in step flow growth, a terrace on the upper surface of thewell layer moves laterally relative to a terrace on the lower surface ofthe well layer, so that a film thickness of the well layer in theinclined region is getting thicker than that of the terrace region otherthan the inclined region. As a result, the band gap energy of theinclined region is smaller than that of the terrace region, and thelocalization of carriers is easy to occur as in the case of the firstGa-rich region of the n-type layer. For this reason, the light emissionin the well layer is more remarkable in the inclined region than in theterrace region. In the above-mentioned Non-Patent Documents 1 and 2, thesame content for the well layer of AlGaN-based semiconductor isreported. Incidentally while in the well layer composed of theAlGaN-based semiconductor, the compositional modulation occurs inaddition to the above film-thickness modulation so that the band gapenergy becomes even smaller than the terrace region, the similarlocalization of carriers occurs even in the well layer in which thecompositional modulation does not occur. Incidentally each terraceregion of the well layer and the barrier layer is a region sandwichedbetween the terrace on the upper surface and the terrace on the lowersurface of each layer in the c-axis direction. Therefore, portion otherthan each terrace region of the well layer and the barrier layer is theboundary region (the inclined region) of each layer.

Furthermore, the multi-step terraces formed by epitaxial growth of theactive layer are formed successively from the multi-step terraces formedby epitaxial growth of the n-type layer. Therefore, the carriers(electrons) supplied to the well layer along the current path in thefirst Ga-rich region are intensively supplied to the boundary region(the inclined region) between adjacent terraces where light emission isconcentrated in the well layer.

Therefore, the n-type AlGaN region, which is the metastable AlGaN havingan AlN mole fraction of 50%, is stably formed in the first Ga-richregion dominantly present in the stratiform region, whereby the carrierscan be stably supplied to the inclined region of the well layer, andvariation in characteristics such as the luminous efficiency of thenitride semiconductor ultraviolet light-emitting element can besuppressed.

Furthermore, since the upper limit of the AlN mole fraction in then-type body region of the n-type layer and the upper limit of the targetAlN mole fraction of the n-type layer are defined to be 66%, themetastable AlGaN in which the AlGaN composition ratio is Al₂Ga₁N₃ is notdominantly formed in the n-type layer. If the upper limit is 67% ormore, the metastable AlGaN of Al₂Ga₁N₃ is stably formed in the n-typebody region, it is getting difficult to sufficiently supply Ga from themetastable AlGaN of Al₂Ga₁N₃ to the first Ga-rich region to stably formthe metastable AlGaN of Al₁Ga₁N₂, an AlN mole fraction of the n-typeAlGaN-based semiconductor formed in the first Ga-rich region will berandomly varied, and the desired effect cannot be expected.

Furthermore, since the n-type layer has an AlN mole fraction of 50% ormore and ultraviolet rays having a peak emission wavelength of 300 nm to327 nm emitted at the well layer of the active layer pass through then-type layer, the element structure for extracting the ultravioletlight-emission from the n-type layer side can be adopted.

Incidentally in order to achieve a peak emission wavelength of 300 nm to327 nm, it is common to configure the well layer with an AlGaN-basedsemiconductor and adjust the AlN mole fraction, for example, within arange of less than 33%. However, in this case, since the compositionratio of Ga is large, the composition fluctuates greatly due to thesegregation of Ga, and degradation of emission characteristics such asvariations in emission wavelength, broadening of emission spectrum, andseparation of peak wavelengths becomes a problem. It was difficult tostably achieve the desired peak emission wavelength.

However, according to the nitride semiconductor ultravioletlight-emitting element having the above-mentioned features, since thewell layer is composed of the GaN-based semiconductor and there is nofluctuation of the composition due to the segregation of Ga, thevariation in characteristics due to the drift of the crystal growthapparatus or the like can be suppressed. Therefore, the variation incharacteristics can be suppressed in the emission wavelength as well.

Furthermore, in the nitride semiconductor ultraviolet light-emittingelement of the above-described feature, it is preferable that the peakemission wavelength is set within a range of 300 nm to 327 nm with athickness of a boundary region between adjacent terraces of themulti-step terraces of the well layer being within a range of 2 unitcells to 4 unit cells in c-axis direction and with AlN mole fractions oftwo AlGaN-based semiconductor layers vertically adjacent to the top andbottom of the well layer being within a range of 50% to 100%.

Furthermore, in the manufacturing method of the nitride semiconductorultraviolet light-emitting element of the above-described feature, it ispreferable that the method comprises setting a thickness of a boundaryregion between adjacent terraces of the multi-step terraces of the welllayer within a range of 2 unit cells to 4 unit cells in c-axis directionand AlN mole fractions of two AlGaN-based semiconductor layersvertically adjacent to the top and bottom of the well layer within arange of 50% to 100% for the peak emission wavelength to be within arange of 300 nm to 327 nm, in the second step, growing the well layer,and in at least one of the first step, the second step, and the thirdstep, growing the two AlGaN-based semiconductor layers verticallyadjacent to the top and bottom of the well layer.

In these preferred implementations, the peak emission wavelength of 300nm to 327 nm is specifically achieved with the well layer being composedof the GaN-based semiconductor.

Incidentally, the two AlGaN-based semiconductor layers verticallyadjacent to the top and bottom the well layer correspond to the n-typelayer and the p-type layer when the active layer is composed of only onewell layer. For example, when the p-type layer is composed of multiplelayers of p-type AlGaN-based semiconductors with different AlN molefractions, the p-type AlGaN-based semiconductor layer in contact withthe well layer corresponds to the p-type layer. When the active layerincludes one or more well layers and a barrier layer in contact with then-type layer side or the p-type layer side of the one or more welllayers, or when the active layer includes two or more well layers and abarrier layer inserted between two adjacent well layers, the barrierlayer corresponds to at least one of the two AlGaN-based semiconductorlayers. Furthermore, in the case where the number of well layers is oneor more, when the well layer closer to the n-type layer and the n-typelayer are in contact with each other, since the n-type AlGaN regions inwhich the AlGaN composition ratio is an integer ratio of Al₁Ga₁N₂ existin the first Ga-rich region, the condition that the AlN mole fraction iswithin the range of 50% to 100% is satisfied.

Incidentally, in these preferred implementations, the range of 2 unitcells to 4 unit cells in c-axis direction for the thickness of theboundary region of the well layer and the range of 50% to 100% for theAlN mole fractions of the two AlGaN-based semiconductor layersvertically adjacent to the top and bottom of the well layer indicatethat the peak emission wavelength within a range of 300 nm to 327 nm canbe achieved by appropriately setting the thickness of the boundaryregion and the AlN mole fractions of the two AlGaN-based semiconductorlayers within the respective ranges, but it does not guarantee that thepeak emission wavelength within the range of 300 nm to 327 nm can beachieved over the entire respective ranges. This point will be describedlater.

Furthermore, in the nitride semiconductor ultraviolet light-emittingelement of the above-described feature, it is preferable that the activelayer has a multi-quantum-well structure including two or more welllayers, and that a barrier layer composed of AlGaN-based semiconductoris present between two of the well layers.

Furthermore, the manufacturing method of the nitride semiconductorultraviolet light-emitting element of the above-described feature, inthe second step, it is preferable to stack a well layer composed of aGaN-based semiconductor and a barrier layer composed of an AlGaN-basedsemiconductor alternately by epitaxial growth, and to form the activelayer of a multi-quantum-well structure including two or more welllayers, in which multi-step terraces parallel to the (0001) plane appearon each surface of the barrier layer and the well layer.

According to these preferred implementations, the active layer has amulti-quantum-well structure, and the luminous efficiency can beexpected to be improved as compared with the luminous efficiency whenonly one well layer is used.

Furthermore, in the nitride semiconductor ultraviolet light-emittingelement of the above-described feature, it is further preferable thatthe barrier layer is composed of an AlGaN-based semiconductor having anAlN mole fraction within a range of 50% to 90%, boundary region partsbetween adjacent terraces of the multi-step terraces of the barrierlayer located at least on the most p-type layer side between two of thewell layers has a second Ga-rich region with a locally lower AlN molefraction within the same barrier layer.

Furthermore, in the manufacturing method of the nitride semiconductorultraviolet light-emitting element of the above-described feature, whenforming the barrier layer composed of an AlGaN-based semiconductor inthe second step, it is more preferable to form a second Ga-rich regionhaving a locally lower AlN mole fraction within the same barrier layerin boundary region parts between the terraces of the barrier layerlocated at least on the most p-type layer side between two of the welllayers.

According to these preferred implementations, the carrier localizationcan occur in the second Ga-rich region of the barrier layer as well asin the first Ga-rich regions of the n-type layer. Therefore, whensupplying the carriers (electrons) from the n-type layer to the boundaryregion (the inclined region) between the adjacent terraces in whichlight emission is concentrated in the well layer, it can be doneefficiently through the first Ga-rich regions of the n-type layer andthe second Ga-rich regions of the barrier layer.

Here, in the multi-quantum-well structure having two or more welllayers, since the emission intensity is large in the well layer at themost p-type layer side, and the second Ga-rich region is formed in thebarrier layer at the n-type layer side of the above-mentioned welllayer, it can be done more efficiently to supply the carriers to thewell layer as described above.

Furthermore, in the nitride semiconductor ultraviolet light-emittingelement of the above preferred implementation, it is preferable that anAlGaN region in which an AlGaN composition ratio is an integer ratio ofAl₁Ga₁N₂, Al₂Ga₁N₃, Al₃Ga₁N₄ or Al₅Ga₁N₆ exists in the second Ga-richregion of the barrier layer.

Furthermore, in the manufacturing method of the nitride semiconductorultraviolet light-emitting element of the above preferredimplementation, it is preferable in the second step,

1) to grow an AlGaN region in which an AlGaN composition ratio is aninteger ratio of Al₁Ga₁N₂ in the second Ga-rich region by setting atarget AlN mole fraction of the barrier layer within a range of 54% to66%, or2) to grow an AlGaN region in which an AlGaN composition ratio is aninteger ratio of Al₂Ga₁N₃ in the second Ga-rich region by setting atarget AlN mole fraction of the barrier layer within a range of 68% and74%, or3) to grow an AlGaN region in which an AlGaN composition ratio is aninteger ratio of Al₃Ga₁N₄ in the second Ga-rich region by setting atarget AlN mole fraction of the barrier layer within a range of 76% to82%, or4) to grow an AlGaN region in which an AlGaN composition ratio is aninteger ratio of Al₅Ga₁N₆ in the second Ga-rich region by setting atarget AlN mole fraction of the barrier layer within a range of 85% to90%.

According to these preferred implementations, since the metastable AlGaNis present in the second Ga-rich region of the barrier layer, thevariation of the AlN mole fraction of the second Ga-rich region issuppressed as well as the first Ga-rich region of the n-type layer andthe region of the metastable AlGaN is stably formed in the secondGa-rich region. Therefore, the effects achieved by the second Ga-richregion of the barrier layer are more stably realized.

Furthermore, it is preferable that the nitride semiconductor ultravioletlight-emitting element of the above features further comprises anunderlying part containing a sapphire substrate, the sapphire substratehas a main surface inclined by a predetermined angle with respect to the(0001) plane, the light-emitting element structure part is formed abovethe main surface, each semiconductor layer at least from the mainsurface of the sapphire substrate to the surface of the active layer isan epitaxially grown layer having a surface on which multi-step terracesparallel to the (0001) plane are formed.

According to the preferred implementation, a miscut sapphire substratecan be used to perform epitaxial growth such that multi-step terracesappear on the surface of each layer from the main surface of thesapphire substrate to the surface of the active layer, thereby realizingthe nitride semiconductor ultraviolet light-emitting element of theabove-described feature.

Effect of the Invention

According to the nitride semiconductor ultraviolet light-emittingelement and the manufacturing method of the nitride semiconductorultraviolet light-emitting element of the above-described features, itis possible to stably provide a nitride semiconductor ultravioletlight-emitting element having a peak emission wavelength within a rangeof 300 nm to 327 nm with the variation in characteristics due to thedrift of the crystal growth apparatus or the like being suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically illustrating the wurtzite crystalstructure of AlGaN.

FIG. 2 is a plan view showing the positional relationship between eachsite of A plane and each site of B plane as viewed from the c-axisdirection of the wurtzite crystal structure shown in FIG. 1 .

FIG. 3 is a diagram schematically showing the arrangement of Al and Gaon A3 plane and B3 plane in each of five combinations of AlGaNcomposition ratios represented by integer ratios.

FIG. 4 is a fragmentary cross-sectional view schematically showing anexemplary configuration of a nitride semiconductor ultravioletlight-emitting element according to an embodiment of the presentinvention.

FIG. 5 is a fragmentary cross-sectional view schematically showing anexemplary laminated structure of an active layer of the nitridesemiconductor ultraviolet light-emitting element shown in FIG. 1 .

FIG. 6 is a graph showing the relationship between the emissionwavelength of the quantum-well structure consisting of GaN well layerand AlGaN barrier layer, the film thickness of the well layer, and theAlN mole fraction of the barrier layer.

FIG. 7 is a plan view schematically showing an exemplary structure whenthe nitride semiconductor ultraviolet light-emitting element shown inFIG. 4 is viewed from the upper side of FIG. 4 .

FIG. 8 is a HAADF-STEM image showing the cross-sectional construction inthe n-type cladding layer.

FIG. 9 is a diagram illustrating five measurement regions A to E inwhich line-analysis of the cross-sectional TEM-EDX in the n-typecladding layer is performed, in the HAADF-STEM image shown in FIG. 8 .

FIG. 10A is a diagram showing measurement results of AlN mole fractionand GaN mole fraction by the line-analysis of the cross-sectionalTEM-EDX in the n-type cladding layer in the measurement region A shownin FIG. 9 .

FIG. 10B is a diagram showing measurement results of AlN mole fractionand GaN mole fraction by the line-analysis of the cross-sectionalTEM-EDX in the n-type cladding layer in the measurement region B shownin FIG. 9 .

FIG. 10C is a diagram showing measurement results of AlN mole fractionand GaN mole fraction by the line-analysis of the cross-sectionalTEM-EDX in the n-type cladding layer in the measurement region C shownin FIG. 9 .

FIG. 10D is a diagram showing measurement results of AlN mole fractionand GaN mole fraction by the line-analysis of the cross-sectionalTEM-EDX in the n-type cladding layer in the measurement region D shownin FIG. 9 .

FIG. 10E is a diagram showing measurement results of AlN mole fractionand GaN mole fraction by the line-analysis of the cross-sectionalTEM-EDX in the n-type cladding layer in the measurement region E shownin FIG. 9 .

FIG. 11 is a SEM image showing the measurement regions of the AlN molefraction by the CL method in the n-type cladding layer.

FIG. 12 is a diagram showing the first and second CL spectra calculatedfrom ten CL spectra measured in each measurement region shown in FIG. 11.

FIG. 13 is a fragmentary cross-sectional view schematically showing anexample of an element structure of a general ultraviolet light-emittingdiode.

DESCRIPTION OF EMBODIMENT

A nitride semiconductor ultraviolet light-emitting element (hereinafter,simply referred to as a “light-emitting element”) according to anembodiment of the present invention will be described with reference tothe drawings. In the drawings as pattern diagram used in the followingdescription, the dimensional ratios of each part are not necessarily thesame as those of the actual elements because the essential part isemphasized to schematically show the invention for ease of understandingof the description. Hereinafter, in the present embodiment, descriptionwill be made on the assumption that the light-emitting element is alight-emitting diode.

First Embodiment <Element Structure of Light-Emitting Element>

As shown in FIG. 4 , the light-emitting element 1 of the presentembodiment includes an underlying part 10 including a sapphire substrate11, and a light-emitting element structure part 20 including a pluralityof AlGaN-based semiconductor layers 21-25, a p-electrode 26, and ann-electrode 27. The nitride semiconductor light-emitting element 1 ismounted (flip-chip mounted) on a mounting base (a submount or the like)with facing a side of the light-emitting element structure part 20 (anupper side in FIG. 4 ) toward the mounting base, and light is extractedfrom a side of the underlying part 10 (a lower side in FIG. 4 ). In thisspecification, for convenience of explanation, a direction perpendicularto the main surface 11 a of the sapphire substrate 11 (or the uppersurface of the underlying part 10 and the respective AlGaN-basedsemiconductor layers 21 to 25) is referred to as “up and down direction”(or “vertical direction”), and a direction from the underlying part 10to the light-emitting element structure part 20 is set to an upwarddirection and an opposite direction thereof is defined as a downwarddirection. A plane parallel to the vertical direction is referred to asa “first plane”. A plane parallel to the main surface 11 a of thesapphire substrate 11 (or the upper surface of the underlying part 10and the respective AlGaN-based semiconductor layers 21 to 25) isreferred to as a “second plane,” and a direction parallel to the secondplane is referred to as a “lateral direction.”

The underlying part 10 is configured with the sapphire substrate 11 andan AlN layer 12 formed directly on the main surface 11 a of the sapphiresubstrate 11. The sapphire substrate 11 is a slightly inclinedsubstrate, in which the main surface 11 a is inclined at an angle(miscut angle) within a certain range (e.g., from 0 degree to about 6degrees) with respect to the (0001) plane and the multi-step terracesappear on the main surface 11 a.

The AlN layer 12 is composed of AlN crystal epitaxially grown on themain surface of the sapphire substrate 11, the AlN crystal has anepitaxial crystal orientation relationship with respect to the mainsurface 11 a of the sapphire substrate 11. Specifically for example, theAlN crystal is grown so that the C-axis direction of the sapphiresubstrate 11 (<0001> direction) and the C-axis direction of the AlNcrystal is aligned. The AlN crystal constituting AlN layer 12 may be anAlN-based semiconductor layer which contains a trace amount of Ga oranother impurity. In the present embodiment, the film thickness of theAlN layer 12 is assumed to be about 2 μm to 3 μm. The structure of theunderlying part 10 and a substrate to be used are not limited to thosedescribed above. For example, an AlGaN-based semiconductor layer havingan AlN mole fraction greater than or equal to the AlN mole fraction ofthe AlGaN-based semiconductor layer 21 may be provided between the AlNlayer 12 and the AlGaN-based semiconductor layer 21.

The AlGaN-based semiconductor layers 21-25 of the light-emitting elementstructure part 20 comprises a structure having an n-type cladding layer21 (n-type layer), an active layer 22, an electron blocking layer 23(p-type layer), a p-type cladding layer 24 (p-type layer), and a p-typecontact layer 25 (p-type layer) stacked in order from the underlyingpart 10 side by epitaxially growing them in order.

In the present embodiment, each semiconductor layer within the AlN layer12 of the underlying part 10, the n-type cladding layer 21 and theactive layer 22 of the light-emitting element structure part 20 whichare epitaxially grown in order from the main surface 11 a of thesapphire substrate 11 has a surface on which multi-step terracesparallel to the (0001) plane originating from the main surface 11 a ofthe sapphire substrate 11 are formed. Since the p-type layers of theelectron blocking layer 23, the p-type cladding layer 24, and the p-typecontact layer 25 are formed on the active layer 22 by epitaxial growth,the same multi-step terraces can be formed, but the p-type layers do notnecessarily need to have surfaces on which the same multi-step terracesare formed.

As shown in FIG. 4 , in the light-emitting element structure part 20,the active layer 22, the electron blocking layer 23, the p-type claddinglayer 24, and the p-type contact layer 25 are formed on the first regionR1 of the upper surface of the n-type cladding layer 21 as a result ofremoving portions of them stacked on the second region R2 of the uppersurface of the n-type cladding layer 21 by etching or the like. Theupper surface of the n-type cladding layer 21 is exposed in the secondregion R2 except for the first region R1. The upper surface of then-type cladding layer 21 may differ in height between the first regionR1 and the second region R2, as schematically shown in FIG. 4 , wherethe upper surface of the n-type cladding layer 21 is individuallydefined in the first region R1 and the second region R2.

The n-type cladding layer 21 is composed of the n-type AlGaN-basedsemiconductor, and stratiform regions, in which AlN mole fraction islocally lower within the n-type cladding layer 21, are present uniformlydispersed in the n-type cladding layer 21. As described above, the firstGa-rich region 21 a, which includes the n-type AlGaN region in which theAlGaN composition ratio is an integer ratio of Al₁Ga₁N₂ (i.e., then-type metastable AlGaN with an AlN mole fraction of 50%), is dominantlypresent in the stratiform regions. FIG. 4 schematically shows thestratiform region which is entirely the first Ga-rich region 21 a, asexamples in which the first Ga-rich region 21 a is dominantly present inthe stratiform region. The region other than the stratiform region inthe n-type cladding layer 21 is referred to as an n-type body region 21b.

In the present embodiment, the AlN mole fraction of the n-type bodyregion 21 b is adjusted within a range of 54% to 66%. As the filmthickness of the n-type cladding layer 21 is assumed to be about 1 μm to2 μm, as with the film thickness adopted in the common nitridesemiconductor ultraviolet light-emitting element, but the film thicknessmay be about 2 μm to 4 μm. In the following, for brevity purposes, then-type AlGaN region of the metastable AlGaN, in which the AlGaNcomposition ratio is an integer ratio of Al₁Ga₁N₂, present in the firstGa-rich region 21 a is referred to as a “metastable n-type region” forconvenience. A region other than the metastable n-type region present inthe first Ga-rich region 21 a, in which the AlN mole fraction slightlyfluctuates with respect to 50% (½), is referred to as a “metastableneighbor n-type region”. Here, the metastable n-type region does notnecessarily need to be continuously stratified in the plurality ofstratiform first Ga-rich regions 21 a and may intermittently existseparated by the metastable neighbor n-type region.

The active layer 22 has a multi-quantum-well structure in which two ormore well layers 220 composed of a GaN-based semiconductor, and one ormore barrier layers 221 composed of an AlGaN-based semiconductor or anAlN-based semiconductor are stacked alternately. The barrier layer 221is not necessarily provided between the lowermost well layer 220 and then-type cladding layer 21. In addition, the barrier layer 221 or a layerof AlGaN or AlN having a thinner thickness and a higher AlN molefraction than the barrier layer 221 may be provided between theuppermost well layer 220 and the electron blocking layer 23.

FIG. 5 schematically shows an exemplary laminated structure of the welllayer 220 and the barrier layer 221 in the active layer 22. FIG. 5illustrates an example for three well layers 220. The structure in whichthe terrace T in the well layer 220 and the barrier layer 221 shown inFIG. 5 grows in multi-steps is a known structure as disclosed inNon-Patent Documents 1 and 2 described above. The boundary region BAbetween the adjacent terraces T has an inclined region inclined withrespect to the (0001) plane as described above. In the presentembodiment, the tread (distance between adjacent boundary regions BA) ofone terrace T is assumed to be several tens nm to several hundreds nm.

In the present embodiment, in order to make a peak emission wavelengthof an ultraviolet light-emission from the well layer 220 within a rangeof 300 nm to 327 nm, a thickness of a boundary region between adjacentterraces of the multi-step terraces of the well layer is adjusted withina range of 2 unit cells to 4 unit cells in c-axis direction and an AlNmole fraction of the barrier layer is adjusted within a range of 50% to100%.

FIG. 6 is a graphical representation of the simulated emissionwavelengths (corresponding to the peak emission wavelengths) for thequantum-well structure model, in which the well layer is composed of GaNand the barrier layer is composed of AlGaN or AlN, obtained by changingthe film thickness of the well layer within a range of 4 ML (monolayer)to 10 ML and setting the AlN mole fraction of the barrier layer to 66.7%(AlGaN) and 100% (AlN).

From FIG. 6 , the quantum confinement effect in the well layer isincreased and the emission wavelength is shortened as the AlN molefraction of the barrier layer 221 is increased and the film thickness ofthe well layer 220 is decreased within a range of 4 ML to 8 ML (2 unitcells to 4 unit cells). It can also be seen that within the aboveranges, the emission wavelength varies approximately in a range of 270nm to 325 nm. Furthermore, If the emission wavelength is desired to be327 nm slightly longer than 325 nm, it can be achieved by making the AlNmole fraction of the barrier layer slightly smaller than 66.7%. It canbe seen from FIG. 6 that the emission wavelength within the range of 300nm to 327 nm can be achieved by adjusting the thickness of the inclinedregion of the well layer within a range of 2 unit cells to 4 unit cellsand the AlN mole fraction of the barrier layer within a range of 50% to100%.

Since the ultraviolet light emission in the well layer 220 is remarkablygenerated in the boundary region (the inclined region) BA, it isessential that the film thickness condition of the well layer 220 issatisfied in the inclined region BA. The film thickness of the terraceregion TA of the well layer 220 is preferably adjusted within the rangeof 2 unit cells to 4 unit cells as in the case of the inclined regionBA, but may partially exceed 4 unit cells due to a hexagonal minuteprojection (hillock) or the like which may exist on a part of theterrace T. It is preferable to adjust the film thickness of the barrierlayer 221, including the terrace region TA and the inclined region BA,within a range of 6 nm to 8 nm, for example.

As described above, the barrier layer 221 has a surface on whichmulti-step terraces T parallel to the (0001) plane are formed as withthe n-type cladding layer 21 and the well layer 220 and is composed ofan AlGaN-based semiconductor as with the n-type cladding layer 21. Here,the barrier layer 221 may be composed of an AlGaN-based semiconductor inwhich the AlN mole fraction is not 100% in accordance with the peakemission wavelength and the film thickness of the inclined region in thewell layer 220, although the barrier layer 221 may be composed of anAlN-based semiconductor in which the AlN mole fraction is 100%.Therefore, as schematically shown in FIG. 5 , when the barrier layer 221is composed of an AlGaN-based semiconductor in which AlN mole fractionis not 100%, the second Ga-rich region 221 a having a locally lower AlNmole fraction in the barrier layer 221 can be formed in the boundaryregion (the inclined region) BA between the adjacent terraces T of thebarrier layer 221 or its vicinity, similarly to the n-type claddinglayer 21. A region other than the second Ga-rich region 221 a in thebarrier layer 221 is referred to as a barrier body region 221 b forconvenience. The barrier body region 221 b mainly exists in the terraceregion TA in the barrier layer 221. If the AlN mole fraction of theentire barrier layer 221 including the second Ga-rich region 221 a is,as an example, in a range of 50% to 90%, which is a part of theabove-mentioned range of 50% to 100%, it is preferable that the AlN molefraction difference between the second Ga-rich region 221 a and thebarrier body region 221 b is 4% to 5% or more in order to sufficientlysecure the effect of the carrier localization in the second Ga-richregion 221 a, but the effect of the carrier localization can be expectedeven if the AlN mole fraction difference is about 1%. Therefore, in thisembodiment, the AlN mole fraction of the barrier body region 221 b isset within a range of 51% to 90%

The electron blocking layer 23 is composed of a p-type AlGaN-basedsemiconductor. The p-type cladding layer 24 is composed of a p-typeAlGaN-based semiconductor. The p-type contact layer 25 is composed of ap-type AlGaN-based semiconductor or p-type GaN based semiconductor. Thep-type contact layer 25 is typically composed of GaN. The thicknesses ofthe respective layers such as the active layer 22, the electron blockinglayer 23, the p-type cladding layer 24, and the p-type contact layer 25are appropriately determined in accordance with the emission wavelengthcharacteristics and the electric characteristics of the light-emittingelement 1. The p-type cladding layer 24 may be omitted in order toreduce the parasitic resistance of the p-type layers.

The p-electrode 26 is made of, for example, a multilayer metal film suchas Ni/Au, and is formed on the upper surface of the p-type contact layer25. The n-electrode 27 is made of, for example, a multilayer metal filmsuch as Ti/Al/Ti/Au and is formed on a part of the exposed surface ofthe n-type cladding layer 21 in second region R2. The p-electrode 26 andthe n-electrode 27 are not limited to the multilayer metal filmdescribed above, and the electrode structure such as the metalconstituting each electrode, the number of layers, and the stackingorder of layers may be changed as appropriate. FIG. 7 shows an exampleof shapes of the p-electrode 26 and the n-electrode 27 viewed from theupper side of the light-emitting elements 1. In FIG. 7 , a line BLexisting between the p-electrode 26 and the n-electrode 27 represents aboundary line between the first region R1 and the second region R2 andcoincides with the outer peripheral side wall surfaces of the activelayer 22, the electron blocking layer 23, the p-type cladding layer 24,and the p-type contact layer 25.

Although in the present embodiment, as shown in FIG. 7 , a comb-likeshape is employed as an example of the planarly-viewed shapes of thefirst region R1 and the p-electrode 26, the planarly-viewed shapes andarrangements of the first region R1 and the p-electrode 26 are notlimited to the illustration shown in FIG. 7 .

When a forward bias is applied between the p-electrode 26 and then-electrode 27, holes are supplied from the p-electrode 26 toward theactive layer 22, electrons are supplied from the n-electrode 27 towardthe active layer 22, and the supplied holes and electrons respectivelyreach the active layer 22 and recombine to emit light. This also causesa forward current to flow between the p-electrode 26 and the n-electrode27.

As to the first Ga-rich region 21 a of the n-type cladding layer 21, asshown schematically by a double line in FIG. 4 , a plurality of layersis vertically separated from each other. Furthermore, in one first plane(for example, the cross section shown in FIG. 4 ) which is parallel tothe vertical direction, at least a part of the extending direction ofthe first Ga-rich region 21 a is inclined with respect to the lateraldirection (the extending direction of an intersection line of the firstplane and the second plane). In the first plane shown in FIG. 4 , eachlayer of the first Ga-rich regions 21 a is schematically illustrated bya parallel line (double line), the inclination angle θ formed betweenthe extending direction and the lateral direction is not necessarily thesame between the first Ga-rich regions 21 a, and may vary depending onthe position even within the same first Ga-rich region 21 a, so that thefirst Ga-rich region 21 a on the first plane do not necessarily extendlinearly. Furthermore, the inclination angle θ is also changed by theorientation of the first plane. Therefore, a part of the first Ga-richregion 21 a may intersect with or diverge from another first Ga-richregion 21 a on the first plane. It is clearly indicated in HAADF-STEMimage shown in FIG. 8 that the inclination angle formed between theextending direction and the lateral direction of the first Ga-richregion 21 a varies depending on the position, and that the first Ga-richregion 21 a is uniformly dispersed in the n-type cladding layer 21.

The first Ga-rich region 21 a is shown as one line (double line) on thefirst plane in FIG. 4 , but also extends in a direction perpendicular tothe first plane in a direction parallel or inclined to the second planeand has a two-dimensional extension. Accordingly the plurality of firstGa-rich regions 21 a can be observed in stripes on the plurality ofsecond planes in the n-type cladding layer 21.

The first Ga-rich region 21 a is a stratiform region with locally lowerAlN mole fraction in the n-type cladding layer 21, as described above.That is, the AlN mole fraction of the first Ga-rich region 21 a isrelatively low with respect to the AlN mole fraction of the n-type bodyregion 21 b. In addition, when the AlN mole fractions of both regionsare asymptotically consecutive in the vicinity of the boundary betweenthe first Ga-rich region 21 a and the n-type body region 21 b, theboundary between both regions cannot be clearly defined.

Therefore, in such cases, a portion in which the AlN mole fraction islower than a reference value can be relatively defined as the firstGa-rich region 21 a, assuming that the reference value is the averageAlN mole fraction of the entire n-type cladding layer 21, for example,the AlN mole fraction serving as a basis for the growth condition of then-type cladding layer 21 (supply amount and flow rate of the sourcegases and the carrier gas used in the metalorganic vapor phase epitaxymethod), which will be described later. In addition to theabove-mentioned defining methods, for example, based on a HAADF-STEMimage to be described later, a portion having a large brightness changemay be defined as the boundary between both layers. However, in thepresent invention, the definition of the boundary between both layers isnot significant, and it is sufficient if the presence of the firstGa-rich region 21 a itself can be grasped.

Indeed, since the first Ga-rich region 21 a is formed with themass-transfer of Ga from the n-type body region 21 b, the average AlNmole fraction in the first Ga-rich region 21 a varies depending on theamount of Ga supplied from the n-type body region 21 b, and the AlN molefraction is not necessarily uniform even in the first Ga-rich region 21a. However, in the present embodiment, since the metastable n-typeregion is stably formed in the first Ga-rich region 21 a, even if thereis a small variation in the above Ga supply amount, the variation isabsorbed by the metastable n-type region, and the variation of the AlNmole fraction in the first Ga-rich region 21 a is suppressed. Therefore,the minimum value of the AlN mole fraction in each of the first Ga-richregions 21 a is 50%, which is the AlN mole fraction of the metastablen-type regions, or a value in the vicinity thereof. However, asdescribed above, the metastable neighbor n-type region also existstogether with the metastable n-type region in the first Ga-rich region21 a. Since the metastable neighbor n-type region is also formed withthe mass-transfer of Ga from the n-type body region 21 b, usually theAlN mole fraction of the metastable neighbor n-type region is higherthan the AlN mole fraction of the metastable n-type region, and theaverage AlN mole fraction in the first Ga-rich region 21 a is slightlyhigher than the AlN mole fraction of the metastable n-type region.

On the other hand, in the n-type body region 21 b, Ga is supplied to thefirst Ga-rich region 21 a, so that the AlN mole fraction is relativelyhigh at a portion in the n-type body region 21 b where Ga has beenmass-transferred. Furthermore, the mass-transfer of Ga that does notlead to the formation of the first Ga-rich region 21 a may also occur inthe n-type body region 21 b, so that the AlN mole fraction also variesto some extent in the n-type body region 21 b. However, as describedabove, since the carriers in the n-type cladding layer 21 is localizedin the first Ga-rich region 21 a with smaller band gap energy than then-type body region 21 b, and in the n-type cladding layer 21, thecurrent flows stably preferentially in the first Ga-rich region 21 a,even if the AlN mole fraction in the n-type body region 21 b variesslightly the characteristics variation of the light-emitting element 1is suppressed by the first Ga-rich region 21 a.

<Method for Manufacturing Light-Emitting Element>

Next, an example of a manufacturing method of the light-emitting element1 illustrated in FIG. 4 will be described.

First, by a well-known epitaxial growth method such as metalorganicvapor phase epitaxy (MOVPE) method, the AlN layer 12 contained inunderlying part 10 and the nitride semiconductor layers 21 to 25contained in the light-emitting element structure part 20 areepitaxially grown on the sapphire substrate 11 sequentially andlaminated. At this time, for example, Si is doped into the n-typecladding layer 21 as a donor impurity and Mg is doped into the electronblocking layer 23, the p-type cladding layer 24, and the p-type contactlayer 25 as an acceptor impurity.

In the present embodiment, in order to make multi-step terraces parallelto the (0001) plane on at least the AlN layers 12, the n-type claddinglayer 21, and the active layer 22 (the well layer 220, the barrier layer221), a slightly inclined substrate is used as the sapphire substrate11, in which the main surface 11 a is inclined at an angle (miscutangle) within a certain range (for example, from 0 degree to about 6degrees) with respect to the (0001) plane, and the multi-step terracesappear on the main surface 11 a.

As a condition of such epitaxial growth, in addition to the use of theslightly inclined (0001) sapphire substrate 11, for example, growth ratein which the multi-step terraces easily appear (specifically forexample, the growth rate achieved by appropriately setting variousconditions such as growth temperature, supply amount and flow rate ofthe source gas and the carrier gas) and the like are included. Note thatthese various conditions may differ depending on the type and structureof the film forming apparatus, and therefore, it is good to actuallymake some samples in the film forming apparatus in order to specifythese conditions.

As growth conditions of the n-type cladding layer 21, a growthtemperature, a growth pressure, and a donor impurity concentration areselected so that the growth start points of the first Ga-rich region 21a are formed on the step portions (boundary region) between themulti-step terraces formed on the upper surface of the AlN layer 12 bythe mass transfer of Ga immediately after the growth start, and thefirst Ga-rich region 21 a can grow obliquely upward by the segregationdue to the mass transfer of Ga in accordance with the epitaxial growthof the n-type cladding layer 21 (the base layer 21 a).

Specifically the growth temperature is preferably 1050° C. or higher atwhich the mass transfer of Ga easily occurs, and the growth temperatureis preferably 1150° C. or lower at which a good n-type AlGaN can beprepared. Furthermore, when the growth temperature exceeds 1170° C., themass-transfer of Ga becomes excessive, and the AlN mole fraction tendsto vary randomly even in the first metastable AlGaN. Therefore, such ahigh growth temperature exceeding 1170° C. is not preferable since it isdifficult to form stably the metastable AlGaN in which the AlN molefraction is 50%. The growth pressure of 75 Torr or less is preferable asthe growth condition of a good AlGaN, and the growth pressure of 10 Torror more is practical as the control limit of the film forming apparatus.The donor impurity density is preferably about 1 ×10¹⁸ to 5×10¹⁸ cm⁻³.The above-described growth temperature, growth pressure, and the likeare examples, and the optimum conditions may be appropriately specifiedaccording to the film forming apparatus to be used.

The supply amount and the flow rate of the source gases(trimethylaluminum (TMA) gas, trimethylgallium (TMG) gas, and ammoniagas) and the carrier gas used in the metalorganic vapor phase epitaxymethod are set according to the average AlN mole fraction Xa of theentire n-type cladding layer 21 as a target value. Here, assuming thatthe average AlN mole fraction of the n-type body region 21 b is Xb (=54%to 66%), and the average AlN mole fraction of the first Ga-rich region21 a, in which the metastable n-type region having the AlN mole fractionof 50% and the metastable neighbor n-type region having the AlN molefraction slightly higher than 50%, is Xc (>50%), and considering themass-transfer of Ga from the n-type body region 21 b to the firstGa-rich region 21 a, Xb>Xa>Xc. However, since the volume ratio of thefirst Ga-rich region 21 a to the entire n-type cladding layer 21 issmall, it can be approximately set as Xa=Xb.

In the first Ga-rich region 21 a, the metastable n-type region havingthe AlN mole fraction of 50% is stably present, and since the targetvalue Xa of the AlN mole fraction of the n-type cladding layer 21 is 54%to 66%, the difference between the AlN mole fraction of 50% in themetastable n-type region and the average AlN mole fraction Xb of then-type body region 21 b (Xb-50%) is stably 4% or more, and the carriersin the n-type layer are localized in the first Ga-rich region 21 ahaving a smaller bandgap energy than the n-type body region 21 b.Furthermore, since the upper limit of the target value Xa is 66%, themetastable AlGaN in which the AlGaN composition ratio is Al₂Ga₁N₃ is notdominantly formed in the n-type body region 21 b. If the upper limit ofthe target value Xa is 67% or more, the metastable AlGaN of Al₂Ga₁N₃ isstably formed in the n-type body region 21 b, and it is difficult tofully supply Ga from the metastable AlGaN of Al₂Ga₁N₃ in order to stablyform the metastable AlGaN (the metastable n-type region) of Al₁Ga₁N₂ inthe first Ga-rich region. Therefore, the metastable n-type region havingthe AlN mole fraction of 50% can be stably formed in the first Ga-richregion 21 a by setting the upper limit of the target value Xa to 66%.

Note that the donor impurity concentration does not necessarily have tobe uniformly controlled in the vertical direction with respect to thefilm thickness of the n-type cladding layer 21. For example, there maybe a low impurity concentration layer in which the impurityconcentration of a predetermined thin film thickness portion in then-type cladding layer 21 is lower than the above-mentioned setconcentration, and is controlled to be, for example, less than 1×10¹⁸cm⁻³, more preferably 1×10¹⁷ cm⁻³ or less. As the film thickness of thelow impurity concentration layer, it is preferably about larger than 0nm and 200 nm or less, more preferably about 10 nm or more and 100 nm orless, and still more preferably about 20 nm or more and 50 nm or less.Furthermore, the donor impurity concentration of the low impurityconcentration layer may be lower than the set concentration, and anundoped layer (0 cm⁻³) may be partially included. Furthermore, it ispreferable that a part of or all the low-impurity-concentration layer ispresent in the upper layer region having a depth of 100 nm or less atlower side from the upper surface of the n-type cladding layer 21.

As described above, when the n-type cladding layer 21 having the firstGa-rich region 21 a and the n-type body region 21 b is formed, theactive layer 22 (the well layer 220, the barrier layer 221), theelectron blocking layer 23, the p-type cladding layer 24, the p-typecontact layer 25, and the like are subsequently formed on the entireupper surface of the n-type cladding layer 21 by a well-known epitaxialgrowth method such as a metalorganic vapor phase epitaxy (MOVPE) method.

In forming the active layer 22, under the growth conditions in which theabove-described multi-step terraces easily appear, the well layer 220 isgrown without supplying the TMA gas so that the film thickness of thewell layer 220 is the thickness predetermined within the range of 2 unitcells to 4 unit cells in order to achieve a peak emission wavelength ofthe ultraviolet light emission within the range of 300 nm to 327 nm. Thebarrier layer 221 is grown using the AlN mole fraction (51% to 90% or100%) set for the barrier body region 221 b as a target value under thegrowth conditions in which the above-described multi-step terraceseasily appear in the same manner as in the n-type cladding layer 21.

Next, by a well-known etching method such as reactive ion etching, thesecond region R2 of the nitride semiconductor layers 21 to 25 stacked inthe above manner is selectively etched until the upper surface of then-type cladding layer 21 is exposed. As a result, the second region R2part of the upper surface of the n-type cladding layer 21 is exposed.Then, the p-electrode 26 is formed on the p-type contact layer 25 in theunetched first region R1 and the n-electrode 27 is formed on the n-typecladding layer 21 in the etched second region R2 by a well-knowndeposition method such as an electron-beam evaporation method. After atleast one of the p-electrode 26 and the n-electrode 27 is formed, heattreatment may be performed by a well-known heat treatment method such asRTA (rapid thermal annealing).

Note that, as an example, the light-emitting element 1 can be used in astate in which it is flip-chip mounted on a base such as a submount andthen sealed by a predetermined resin such as a silicone resin or anamorphous fluororesin (e.g., a resin having a lens shape)

<Cross-Sectional Observation and Compositional Analyses of n-TypeCladding Layer>

Next, a sample for observing the cross section of the n-type claddinglayer 21 is produced, a sample piece having a cross sectionperpendicular (or substantially perpendicular) to the upper surface ofthe n-type cladding layer 21 is processed by a focused ion beam (FIB),and the sample piece is observed by a scanning transmission electronmicroscope (STEM), and the result will be described with reference tothe drawings.

The sample was produced by sequentially depositing the n-type claddinglayer 21, the active layer 22, an AlGaN layer having a higher AlN molefraction than the n-type cladding layer 21, an AlGaN layer forprotecting the sample surfaces, and a protective resin film on theunderlying part 10 composed of the sapphire substrate 11 and the AlNlayer 12 in accordance with the manufacturing procedure of the n-typecladding layer 21 and the like described above. In the preparation ofthe sample, the underlying part 10 in which multi-step terraces wereappeared on the surface of the AlN layer 12 using the sapphire substrate11 whose main surface has a miscut angle with respect to the (0001)plane was used. In the preparation of the sample, the film thickness ofthe n-type cladding layer 21 was set to 2 μm, and the target AlN molefraction of the n-type cladding layer 21 was set to 58%. In addition,the dose of donor impurity (Si) was controlled so that the donorimpurity concentration was about 3×10¹⁸ cm⁻³.

FIG. 8 shows the high-angle annular dark-field (HAADF)-STEM image of thecross section of the sample piece. FIG. 8 is a HAADF-STEM image of theentire n-type cladding layer 21 including a part of the sample piecefrom the upper layer of the AlN layer 12 to the upper surface of then-type cladding layer 21.

In HAADF-STEM image, contrasts proportional to atomic weights areobtained, and heavy elements are displayed brightly. Therefore, as tothe first Ga-rich region 21 a and the n-type body region 21 b in then-type cladding layer 21, the first Ga-rich region 21 a having lower AlNmole fraction are displayed brighter than the n-type body region 21 b.HAADF-STEM image is more suitable for observing differences in AlN molefraction than the normal STEM image (bright-field image).

It can be seen from FIG. 8 that a plurality of the first Ga-rich regions21 a, which are stratiform regions having locally lower AlN molefraction, are present dispersed in the vertical direction in the n-typecladding layer 21, and each of the first Ga-rich regions 21 a isextending in a direction inclined with respect to the intersection linebetween the upper surface of the n-type cladding layer 21 and the firstplane on a viewing surface of the HAADF-STEM image (a cross section ofthe sample piece, corresponding to the first plane). Although each ofthe first Ga-rich regions 21 a is extending obliquely upward in a linearmanner, it is not necessarily extending in a straight line, and theinclination angle with respect to the intersection varies by locationeven in the same first Ga-rich region 21 a. On the cross section shownin FIG. 8 (corresponding to the first plane), it is also observed that aportion of the first Ga-rich region 21 a intersects with or branchesfrom another first Ga-rich region 21 a.

In the present embodiment, the compositional analyses in the n-typecladding layer 21 of the sample pieces were performed by two types ofanalysis methods (line analysis of energy dispersive X-ray spectroscopy(cross-sectional TEM-EDX) and CL (cathodoluminescence) method).

In the compositional analysis by EDX method (EDX measurement), anelectron beam probe (diameter: about 2 nm) was scanned firstlongitudinally (vertical direction) and laterally (direction parallel tothe second plane) in the entire measurement region coveringsubstantially the entire area of HAADF-STEM image shown in FIG. 8 , andthe detection data (X-ray intensity corresponding to the respective Aland Ga compositions) at each probe location distributed in a matrix of512×512 at about 4 nm intervals in the longitudinal and transversedirections, was obtained.

Next, in order to perform the line-analysis by the EDX measurement tothe first Ga-rich region 21 a dispersed in the entire measurementregion, five measurement regions A to E having a generally square shape(about 420 nm×about 420 nm) were set in the entire measurement region asshown in FIG. 9 . FIG. 9 shows a rectangular frame representingrespective measurement regions A to E superimposed on HAADF-STEM imageof FIG. 8 . Each of the five measurement regions is set so that at leastone of the first Ga-rich regions 21 a confirmed on HAADF-STEM image iscrossing in each measurement region. The inclination of each measurementregion is set for each measurement region so that the extendingdirection of at least one of the first Ga-rich regions 21 a in themeasurement region is perpendicular to the scanning direction of theline analysis. Each inclination of the measurement regions A to E (theangle between the longitudinal direction of the entire measurementregion and the longitudinal direction of each measurement region) isapproximately equal at about 20°, but strictly speaking, is notnecessarily the same. Here, for convenience of explanation, the scanningdirection of the line analysis is defined as a longitudinal directionand the direction perpendicular to the scanning direction is defined asa transverse direction in each of the measurement regions A to E of FIG.9 , apart from the longitudinal and transverse directions of the entiremeasurement region. The center vertical line shown in each of themeasurement regions indicates the scanning direction, and the centerhorizontal line indicates the position where said at least one of thefirst Ga-rich regions 21 a is assumed to be present and is the origin (0nm) of the scanning position of the line analysis in the compositionalanalysis described later. Incidentally the vertical line indicating thescanning direction is marked with an arrow, which points to thedirection of the AlN layer 12. The scanning positions are set on thecenter vertical line in the longitudinal direction across theabove-mentioned origin at intervals of about 5 nm and within the rangeof 49 to 88 points in total for each measurement region A to E.

In the EDX measurement, since the diameter of the electron beam probe tobe irradiated is as small as about 2 nm, the spatial resolution is high,but since the X-rays emitted from respective probe positions are weak,in the line analysis of the present embodiment, respective detectiondata obtained from a plurality of probe positions aligned in thetransverse direction at each scanning position are accumulated to be thedetection data at each scanning position. Incidentally, “aligned in thetransverse direction” means that the irradiation range of the electronbeam probe is overlapped with the horizontal line intersecting thevertical line and extending in the transverse direction at each scanningposition.

Therefore, when all the plurality of probe positions aligned in thetransverse direction are located within the metastable n-type region ofthe first Ga-rich region 21 a at a certain scanning position, theaccumulated detection data accurately indicates the AlN mole fraction ofthe metastable n-type region. Similarly, when all the plurality of probepositions aligned in the transverse direction are located within then-type body region 21 b at a certain scanning position, the accumulateddetection data accurately indicates the AlN mole fraction of the n-typebody region 21 b.

However, at a certain scanning position, when a part of the plurality ofprobe positions aligned in the transverse direction, or a part of theprobe range (about 2 nm in diameter) of each probe position is locatedwithin the metastable neighbor n-type region or the n-type body region21 b other than the metastable n-type region, due to the extendingdirection of the metastable n-type region of the first Ga-rich region 21a being not exactly perpendicular to the scanning direction of the lineanalysis, or the extending direction of the metastable n-type region ofthe first Ga-rich region 21 a being not straight such as by bending, orthe like, the accumulated detection data indicates an average AlN molefraction of the plurality of probe positions, and indicates a valuehigher than the AlN mole fraction of the metastable n-type region.

Similarly, even if most of the plurality of probe positions aligned inthe transverse direction are located within the n-type body region 21 bat a certain scanning position, when a part of the plurality of probepositions or a part of the probe range (about 2 nm in diameter) of eachprobe position is located in a region having locally lower or higher AlNmole fraction caused by the mass-transfer in the n-type body region 21b, or in a region having locally lower AlN mole fraction other than then-type body region 21 b (a stratiform region other than the firstGa-rich region 21 a, the metastable n-type region or the metastableneighbor n-type region in the first Ga-rich region 21 a), theaccumulated detection data indicates an average AlN mole fraction of theplurality of probe positions, and indicates a value lower or higher thanthe average AlN mole fraction of the n-type body region 21 b (z targetvalue of the AlN mole fraction of the n-type cladding layer 21).

FIG. 10A to FIG. 10E show the results of compositional analyses in then-type cladding layer 21 in the five measurement regions A to E shown inFIG. 9 by the linear analysis of the EDX measurement. In the graphs ofthe compositional analysis results of the measurement regions A to Eshown in FIGS. 10A to 10E, the horizontal axis indicates the scanningposition along the center vertical line of each measurement region, andthe vertical axis indicates the measurement results of the AlN molefraction and the GaN mole fraction. 0 nm of the scanning position of thehorizontal axis indicates the position of the center horizontal lineshown in each measurement region (where at least one of the firstGa-rich regions 21 a is assumed to be present). The scanning positionsare indicated by positive values below the origin (0 nm) (the AlN layer12 side) and by negative values above the origin (the active layer 22side), respectively.

In the EDX measurement, as described above, since the X-rays emittedfrom the probe positions are weak, even if the respective detection data(X-ray intensity of each composition) of the probe positions areaccumulated in the transverse direction at each scanning position, themeasurement error is generally large. For example, when the calibrationis performed based on the AlN mole fraction (100%) of the AlN layer 12where the AlN mole fraction is predetermined, the measurement error ofthe detection data at each scanning position is about ±2 to 3% even nearthe AlN layer 12 as a reference, and the measurement accuracy furtherdecreases with increasing distance from the AlN layer 12. Therefore, inthe present embodiment, in order to suppress the measurement error atthe respective scanning positions to about ±2 to 3% even in the regionapart from the AlN layer 12, the same sample as the sample piece usedfor the EDX measurement was used to perform the compositional analysisof Al and Ga in n-type cladding layer 21 by the Rutherfordbackscattering (RBS) analysis method, and the result obtained by the EDXmeasurement was calibrated using the RBS analysis result. The AlN molefraction and the GaN mole fraction of the measurement regions A to Eshown in FIGS. 10A to 10E show the calibrated results.

From FIG. 10A, in the measurement region A, the presence of the firstGa-rich region 21 a can be confirmed in the region A1 at the scanningpositions of about −131 nm to about −92 nm and in the region A2 at thescanning positions of about −5 nm to about 10 nm. The AlN mole fractionsat nine scanning positions in region A1 are within 49.4% to 52.8% (sixof them are within 49% to 52%, three of them are within 49% to 51%).Region A1 includes the scanning position at which the AlN mole fractionis 50%. The AlN mole fractions at four scanning positions in region A2are within 51.3% to 52.8% (two of them are within 50% to 52%).

From FIG. 10B, in the measurement region B, the presence of the firstGa-rich region 21 a can be confirmed in the region B1 at the scanningpositions of about −116 nm to about −73 nm and in the region B2 at thescanning positions of about −5 nm to about 5 nm. The AlN mole fractionsat ten scanning positions in region B1 are within 51.3% to 52.8% (fourof them are within 50% to 52%). The AlN mole fractions at three scanningpositions in region B2 are within 50.5% and 51.9% (three of them arewithin 50% to 52%).

From FIG. 10C, in the measurement region C, the presence of the firstGa-rich region 21 a can be confirmed in the region C1 at the scanningpositions of about −140 nm to about −111 nm and in the region C2 at thescanning positions of about −10 nm to about 48 nm. The AlN molefractions at seven scanning positions in region C1 are within 51.1% to52.7% (four of them are within 50% to 52%). The AlN mole fractions at 13scanning positions in region C2 are 51.1% and 52.7% (nine of them arewithin 50% to 52%).

From FIG. 10D, in the measurement region D, the presence of the firstGa-rich region 21 a can be confirmed in the region D1 at the scanningpositions of about −160 nm to about −140 nm, in the region D2 at thescanning positions of about −73 nm to about 158 nm, in the region D3 atthe scanning positions of about 0 nm to about 10 nm, and in the regionD4 at the scanning positions of about 97 nm to about 106 nm. The AlNmole fractions at five scanning positions in the region D1 are within49.1% to 52.3% (three of them are within 49% to 52%) and the region D1includes the scanning position at which the AlN mole fraction is 49.8%.The AlN mole fractions at four scanning positions in the region D2 arewithin 51.4% to 52.7% (three of them are within 50% to 52%). The AlNmole fractions at four scanning positions in the region D3 are within51.6% to 52.2% (two of them are within 50% to 52%). The AlN molefractions at three scanning positions in the region D4 are within 51.0%to 51.3% (three of them are within 50% to 52%).

From FIG. 10E, in the measurement region E, the presence of the firstGa-rich region 21 a can be confirmed in the region E1 at the scanningpositions of about −169 nm to about −97 nm and in the region E2 at thescanning positions of about −5 nm to about 5 nm. The AlN mole fractionsat 16 scanning positions in region E1 are within 50.5% to 52.5% (elevenof them are within 50% to 52%, six of them are within 50% to 51%). TheAlN mole fractions at three scanning positions in region E2 are within50.8% to 52.6% (two of them are within 50% to 52%).

As observed above, it is possible to confirm the presence of themetastable n-type regions having the AlN mole fraction of 50% in thefirst Ga-rich region 21 a in the respective regions A1, A2, B1, B2, C1,C2, D1 to D4, E1, and E2 of the measurement regions A to E, consideringthe measurement error of about ±2 to 3% at each scanning position andthe possibility that the average AlN mole fraction of the plurality ofprobe positions aligned in the transverse direction with respect to thefirst Ga-rich region 21 a is higher than the AlN mole fraction of themetastable n-type region. Furthermore, the first Ga-rich regions 21 aare present respectively in the measurement regions A and B of the upperportion close to the upper surface of the n-type cladding layer 21, inthe measurement region C of the central portion, and in the measurementregions D and E of the lower portion close to the AlN layer 12, and itcan be seen that the first Ga-rich regions 21 a are uniformly dispersedin the n-type cladding layer 21.

Furthermore, from FIGS. 10A to 10E, it can be confirmed that the AlNmole fractions in the n-type body regions 21 b adjacent to the regionsA1, A2, B1, B2, C1, C2, D1 to D4, E1, and E2 of the measurement regionsA to E are within the range of about 55% to about 58%. As describedabove, since the target value of the AlN mole fraction of the n-typecladding layer 21 of the sample used for the EDX measurement is 58%, itcan be seen that the FIGS. 10A to 10E accurately represent the AlN molefraction of the n-type body region 21 b, considering the measurementerror of about ±2 to 3% at each scanning position and the possibilitythat the average AlN mole fraction of the plurality of probe positionsaligned in the transverse direction with respect to the n-type bodyregion 21 b is higher or lower than the average AlN mole fraction of then-type body region 21 b.

Next, the measurement results of the AlN mole fractions of the firstGa-rich region 21 a and the n-type body region 21 b in the n-typecladding layer 21 by the CL (cathodoluminescence) method are described.The sample pieces used for the measurement was prepared in the samemanner as the sample pieces used for the observation of HAADF-STEM imageshown in FIG. 8 .

FIG. 11 is a scanning electron microscopy (SEM) image showing a crosssection in the n-type cladding layer 21 of the sample piece. Themeasurement regions (a to d) surrounded by dotted lines in the crosssection indicate the incident areas of the electron beam irradiated forthe measurement, respectively. The measurement regions a and b arelocated at about 1000 nm from the upper surface of the AlN layer 12, andthe measurement regions c and d are located at about 350 nm from theupper surface of the AlN layer 12. Within each measurement region, theelectron beam having a beam diameter of 50 nm was moved laterally andirradiated once each at 50 nm intervals for a total of 10 times tomeasure the CL spectrum at each irradiation.

FIG. 12 shows the first CL spectrum and the second CL spectrum for eachmeasurement region (a to d). The first CL spectrum was obtained byaveraging two CL spectra whose wavelength distributions are closer tothe short wavelength among the ten CL spectra in each measurement region(a to d) and the second CL spectrum was obtained by averaging two CLspectra whose wavelength distributions are closer to the long wavelengthamong the ten CL spectra in each measurement region (a to d).

Since the distance between both ends of the ten electron-beam centers ineach measurement region (a to d) is 450 nm, both the first Ga-richregion 21 a and the n-type body region 21 b are present in the tenirradiation areas. Since the volume ratio of the first Ga-rich region 21a to the entire n-type cladding layer 21 is small, the first CL spectrummainly shows the CL spectrum of the n-type body region 21 b. On theother hand, the second CL spectrum includes the CL spectrum of the firstGa-rich region 21 a. However, since the width in a cross sectionperpendicular to the extending direction of the first Ga-rich region 21a is about 20 nm on average, the n-type body region 21 b may bepartially included within the irradiation range of the beam diameter 50nm. Therefore, the second CL spectrum is a composite spectrum of the CLspectrum of the first Ga-rich region 21 a and the CL spectrum of then-type body region 21 b. However, if the center of each electron beam ofthe two CL spectra whose wavelength distributions are closer to the longwavelength is located at the widthwise center of the first Ga-richregion 21 a, it is likely that the electron beam of the central portionwithin the irradiation range will gather in the first Ga-rich region 21a with lower energy level to excite the first Ga-rich region 21 aexclusively, and it is considered that the second CL spectrum mainlyshows the CL spectrum of the first Ga-rich region 21 a.

Here, the reason why the first CL spectrum is the average of the two CLspectra whose wavelength distributions are closer to the shortwavelength and the second CL spectrum is the average of the two CLspectra whose wavelength distributions are closer to the long wavelengthis as follows. The irradiation positions of the electron beam at eachmeasurement region are set at random, so that the irradiation ranges ofone CL spectrum closest to the short wavelength and the one closest tothe long wavelength are different for each measurement region, and themeasurement results are largely varied for each measurement region.Also, it may be difficult to sort out one CL spectrum closest to theshortest wavelength and the one closest the long wavelength. Therefore,it was decided to mechanically select two CL spectra whose wavelengthdistributions are closer to the short wavelength and the other two CLspectra whose wavelength distributions are closer to the longwavelength, respectively and to take the averages of the respective twoCL spectra in order to suppress the variation for each measurementregion.

First, the first CL spectrum of each measurement region (a to d) will bediscussed. In the measurement region a, a peak of emission wavelengthexists near about 260 nm. In the measurement region b, gradual peaks ofemission wavelength exist at two locations near about 260 nm and nearabout 269 nm. In the measurement region c, a peak of emission wavelengthexists near about 259 nm. In the measurement region d, a peak ofemission wavelength exists near about 258 nm.

The peak wavelength of about 258 nm to about 260 nm in each measurementregion (a to d) corresponds to about 59% to about 61% in terms of theAlN mole fraction, and the above CL wavelength of the first CL spectrumgenerally coincides with the average AlN mole fraction Xb (˜target value58%) of the n-type body region 21 b in consideration of a measurementerror of about ±3% in terms of the AlN mole fraction.

In addition, in the first CL spectra of the measurement regions a, c andd, the long wavelength component longer than a peak wavelength of about258 nm to about 260 nm is larger than the short wavelength componentshorter than the same peak wavelength, and it can be seen that themass-transfer of Ga occurs within two irradiation ranges correspondingto the first CL spectrum of each measurement region. Furthermore, thepeak wavelength of about 269 nm of the first CL spectrum in themeasurement region b corresponds to about 54%±3% in terms of the AlNmole fraction, and generally coincides with the CL wavelength from themetastable neighbor n-type region, whose AlN mole fraction is slightlyhigher than 50%, in the first Ga-rich region 21 a. Therefore, a portionof the two irradiation ranges corresponding to the first CL spectrum ofthe measurement region b includes the first Ga-rich region 21 a formedby the mass-transfer of Ga.

Next, the second CL spectrum of each measurement region (a to d) will bediscussed. In the measurement region a, gradual peaks of emissionwavelength exist at two locations near about 262 nm and near around 270nm. In the measurement region b, a gradual peak of emission wavelengthexists within a range of about 270 nm to around 273 nm. In themeasurement region c, a gradual peak of emission wavelength existswithin a range of about 266 nm to around 269 nm, and the overallplateau-like peak region extends in the range of about 261 nm to about270 nm. In the measurement region d, gradual peaks of emissionwavelengths exist near about 259 nm and near around 268 nm, and theoverall plateau-like peak region extends in the range of about 258 nm toabout 269 nm.

The peak wavelength of about 270 nm to about 273 nm in the measurementregions a and b corresponds to about 51% to about 53% in terms of theAlN mole fraction, and generally coincides with the CL wavelength (about275 nm) corresponding to the metastable n-type region having the AlNmole fraction of 50% present in the first Ga-rich region 21 a inconsideration of a measurement error of about ±3% in terms of AlN molefraction. The second CL spectra of the measurement regions a and b alsoinclude the CL wavelength of about 275 nm, which corresponds to themetastable n-type region, with an emission intensity of about 72 to 78%of the peak intensity. However, the peak wavelength of about 270 nm toabout 273 nm is about 2 to 5 nm shorter than the CL wavelength (about275 nm) corresponding to metastable n-type region. In addition, in eachof the second CL spectra of the measured regions a and b, a gradual peakor shoulder (undulation) of the emission wavelength is present aroundabout 262 nm. These indicate that each of the second CL spectra of themeasured regions a and b appears as a composite spectrum of therespective CL spectra of the metastable n-type region and the metastableneighbor n-type region having slightly higher AlN mole fraction than themetastable n-type region in the first Ga-rich region 21 a and the CLspectrum of the n-type body region 21 b. Furthermore, in the second CLspectrum of the measurement region a, the proportion of the CL spectrumof the n-type body region 21 b in the composite spectrum is larger thanthat of the measurement region b.

On the other hand, in the second CL spectra of the measurement regions cand d, a long-wavelength end of the plateau-like peak region is about269 nm to 270 nm, which corresponds to about 53% to 54% in terms of theAlN mole fraction, and generally coincides with the CL wavelength (about275 nm) corresponding to the metastable n-type region having the AlNmole fraction of 50% present in the first Ga-rich region 21 a inconsideration of a measurement error of about ±3% in terms of AlN molefraction. The second CL spectra of the measurement regions c and dinclude the CL wavelength of about 275 nm corresponding to themetastable n-type region with an emission intensity of about 49% to 55%of the peak intensity. These indicate that the second CL spectra of themeasurement regions c and d appear as the composite spectrum of therespective CL spectra of the metastable n-type region and the metastableneighbor n-type region in the first Ga-rich region 21 a and the CLspectrum of the n-type body region 21 b, as with the measurement regionsa and b. In the second CL spectra of the measurement regions c and d,the proportion of the CL spectrum of the n-type body region 21 b in thecomposite spectrum is larger than that of the measurement regions a andb.

As described above, from the first CL spectra in the respectivemeasurement regions a to d shown in FIG. 12 , the AlN mole fraction ofthe n-type body region 21 b almost coincides with the target value 58%of the AlN mole fraction of the n-type cladding layer 21. Furthermore,from the second CL spectra in the respective measurement regions a to d,it can be seen that the first Ga-rich region 21 a includes not only themetastable n-type region having the AlN mole fraction of 50% and alsothe metastable neighbor n-type region having the AlN mole fractionhigher than that of the metastable n-type region. Furthermore, theanalysis results shown in the first and second CL spectra in themeasurement regions a to d shown in FIG. 12 are in good agreement withthe analysis results obtained by the EDX measurement shown in FIGS. 10Ato 10E, although there are differences in the spatial resolution and thelike due to differences in analysis methods.

It should be noted that, from the second CL spectra in the respectivemeasurement regions a to d shown in FIG. 12 , the abundance ratio of themetastable n-type region in the first Ga-rich region 21 a tends tochange depending on the position in n-type cladding layer 21. However,since there are many uncertainties, detailed examination is omitted.

Here, even if the abundance ratio of the metastable n-type region issmall at the region close to the AlN-layer 12 in the n-type claddinglayer 21, the effect of the present invention is not necessarilyreduced. As described above, the carriers (electrons) in the n-typecladding layer 21 are localized in the first Ga-rich region 21 a, sothat the current can flow preferentially through the first Ga-richregion in the n-type cladding layer 21 stably thereby suppressingvariation in the properties of the light-emitting element. However,since the active layer 22, which is a light-emitting region, is locatedabove the n-type cladding layer 21, the localization is remarkable inthe vicinity of the upper surface of the n-type cladding layer 21 whichis in contact with the active layer 22. Therefore, even if the abovelocalization is insufficient in the area close to the AlN-layer 12 inthe n-type cladding layer 21, it is possible to suppress the variationin the properties of the light-emitting element in the same manner.Further, in the element configuration shown in FIG. 4 , since theforward current flows more in the upper layer side than in the lowerlayer side in the n-type cladding layer 21, it is considered that thereis almost no effect of the insufficient localization in the area closeto the AlN layer 12 in the n-type cladding layer 21.

Second Embodiment

In the first embodiment, when the barrier layer 221 is composed of anAlGaN based semiconductor having an AlN mole fraction of not 100%, it isdescribed as an example that the AlN mole fraction of the entire barrierlayer 221 including the second Ga-rich region 221 a is in the range of50% to 90%, the AlN mole fraction of the barrier body region 221 b is inthe range of 51% to 90%, and the difference in the AlN mole fractionbetween the second Ga-rich region 221 a and the barrier body region 221b is 1% or more in order to ensure the effectiveness of the localizationof carriers in the second Ga-rich region 221 a.

In the second embodiment, similarly to the first Ga-rich regions 21 a ofthe n-type cladding layer 21 in the first embodiment, the second Ga-richregion 221 a of the barrier layer 221 is also preferable to be composedof the metastable AlGaN. Here, since the AlN mole fraction of the entirebarrier layer 221 is in the range of 50% to 90%, in the first metastableAlGaN applicable to the second Ga-rich region 221 a, the AlGaNcomposition ratio is an integer ratio of Al₁Ga₁N₂, Al₂Ga₁N₃ or Al₅Ga₁N₆.Although the second metastable AlGaN of Al₇Ga₅N₁₂ or Al₃Ga₁N₄ is alsoconsidered to be applicable to the second Ga-rich region 221 a, it ispreferable to use a more stable Al₃Ga₁N₄ if the second metastable AlGaNis daringly used. In the second metastable AlGaN of Al₁₁Ga₁N₁₂, sincethe composition ratio of Al is too high, before easy-to-move Ga enterssites which compose a symmetric arrangement, the quantitatively largeamount of Al randomly enters the sites, so that the atomic arrangementof Al and Ga is likely not to be a symmetric arrangement and is close tothe random state, and the above stability is reduced. Therefore, it isconsidered difficult to apply this second metastable AlGaN to the secondGa-rich region 221 a,

When the second Ga-rich region 221 a is composed of a metastable AlGaNof Al₁Ga₁N₂, Al₂Ga₁N₃, Al₃Ga₁N₄, or Al₅Ga₁N₆, the AlN mole fraction ofthe barrier body region 221 b is preferably in a range of 51% to 66%,68% to 74%, 76% to 82%, or 85% to 90%, depending on the four AlN molefractions of the second Ga-rich region 221 a. Here, when the secondGa-rich region 221 a is composed of the metastable AlGaN of Al₅Ga₁N₆, itis preferable to set the AlN mole fraction of the barrier body region221 b not to exceed 90% in order to prevent low stable Al₁Ga₁N₁₂ beingrandomly mixed.

In the manufacturing method of the second Ga-rich region 221 a and thebarrier body region 221 b in the barrier layer 221, as described above,the barrier layer 221 is grown under the growth conditions in whichmulti-step terraces easily appear using the AlN mole fraction set forthe barrier body region 221 b as a target value in the same manner asthe n-type cladding layer 21.

When the first metastable AlGaN of Al₁Ga₁N₂ is grown in the secondGa-rich region 221 a, the target value Xd of the AlN mole fraction ofthe barrier layer 221 is set within the range of 51% to 66% as well asthe target value Xa of the AlN mole fraction of the n-type claddinglayer 21. Similarly when the first metastable AlGaN of Al₂Ga₁N₃ is grownin the second Ga-rich region 221 a, the target value Xd of the AlN molefraction of the barrier layer 221 is set within the range of 68% to 74%,and when the second metastable AlGaN of Al₃Ga₁N₄ is grown in the secondGa-rich region 221 a, the target value Xd of the AlN mole fraction ofthe barrier layer 221 is set within the range of 76% to 82%, and whenthe first metastable AlGaN of Al₅Ga₁N₆ is grown in the second Ga-richregion 221 a, the target value Xd of the AlN mole fraction of thebarrier layer 221 is set within the range of 85% to 90%.

Therefore, the target value Xd of the AlN mole fraction of the barrierlayer 221 is set within a range of 1% or higher than the AlN molefraction of the metastable AlGaN formed in the second Ga-rich region 221a (target metastable AlGaN) and less than the AlN mole fraction of themetastable AlGaN which is nearest to and larger than that of the targetmetastable AlGaN. Therefore, as with the first Ga-rich region 21 a ofthe n-type cladding layer 21, the target metastable AlGaN can be stablyformed in the second Ga-rich region 221 a, and 1% or more of the AlNmole fraction difference between the second Ga-rich region 221 a and thebarrier body region 221 b is ensured, and the carriers in the barrierlayer 221 are localized in the second Ga-rich region 221 a having asmaller band gap energy than the barrier body region 221 b.

Because the second Ga-rich region 221 a is composed of highly stablemetastable AlGaN, variation in the mole fraction of mixed crystalscaused by the drift of the crystal growth apparatus or the like issuppressed, and the second Ga-rich region 221 a in which carrierlocalization occurs in the barrier layer 221 is stably formed with theAlN mole fraction corresponding to the metastable AlGaN used.Consequently the current can stably flow preferentially through thesecond Ga-rich region 221 a even in the barrier layer 221 as in then-type cladding layer 21, and the variation in the properties of thelight-emitting element 1 can be suppressed.

Other Embodiments

Modifications of the first and second embodiments will be describedbelow.

(1) In the first and second embodiments, the active layer 22 was assumedto be composed of the multi-quantum-well structure in which the welllayer 220 and the barrier layer 221 are stacked alternately where thewell layer 220 includes two or more layers composed of a GaN-basedsemiconductor and the barrier layer 221 includes one or more layerscomposed of an AlGaN-based semiconductor or an AlN-based semiconductor.However, the active layer 22 may be configured with a singlequantum-well structure having only one layer of the well layer 220, andit may be configured not to include the barrier layer 221 (quantumbarrier layer). It is obvious that the advantages of the n-type claddinglayer 21 adopted in the above-described embodiments can be obtained inthe same manner for such a single-quantum-well structure.

(2) In the above embodiments, as an example of the growth condition ofthe n-type cladding layer 21, the supply amount and the flow rate of thesource gases and the carrier gas used in the MOVPE method are set inaccordance with the average AlN mole fraction of the entire n-type AlGaNlayer constituting the n-type cladding layer 21. That is, when theaverage AlN mole fraction of the entire n-type cladding layer 21 is setto a constant value vertically it is assumed that the supply amount andthe flow rate of the source gases and the like are controlled to beconstant. However, the supply amount and the flow rate of the sourcegases and the like are not necessarily controlled to be constant.

(3) In the above embodiments, the planarly-viewed shapes of the firstregion R1 and the p-electrode 26 are exemplarily a comb-like shape, butthe planarly-viewed shapes are not limited to the comb-like shape. Inaddition, a plurality of the first regions R1 may be present, and eachof them may be formed in a planarly-viewed shape surrounded by onesecond region R2.

(4) In the above embodiments, the case using the underlying part 10, inwhich the sapphire substrate 11 in which the main surface has a miscutangle with respect to the (0001) surface is used and the multi-stepterraces appear on the surface of the AlN layer 12, is exemplified.However, the magnitude of the miscut angle and the direction in whichthe miscut angle is provided (specifically the direction in which the(0001) surface is inclined, for example, the m-axis direction, thea-axis direction, and the like) may be arbitrarily determined as long asthe multi-step terraces appear on the surface of the AlN layer 12 andthe growth start points of the first Ga-rich region 21 a are formed.

(5) In the above embodiments, as illustrated in FIG. 1 , thelight-emitting element 1 having the underlying part 10 including thesapphire substrate 11 is illustrated as the light-emitting element 1,but the sapphire substrate 11 (or a part or all of layers included inthe underlying part 10) may be removed by lift-off or the like.Furthermore, the substrate constituting the underlying part 10 is notlimited to the sapphire substrate.

INDUSTRIAL APPLICABILITY

The present invention is available to a nitride semiconductorultraviolet light-emitting element which comprises an active layerhaving a quantum-well structure having one or more well layers composedof a GaN-based semiconductor and has a peak emission wavelength within arange of 300 nm to 327 nm.

DESCRIPTION OF SYMBOLS

-   -   1 Nitride semiconductor ultraviolet light-emitting element    -   10 underlying part    -   11 sapphire substrate    -   11 a main surface of sapphire substrate    -   12 AlN layer    -   20 light-emitting element structure part    -   21 n-type cladding layer (n-type layer)    -   21 a first Ga-rich region (n-type layer)    -   21 b n-type body region (n-type layer)    -   22 active layer    -   220 well layer    -   221 barrier layer    -   221 a second Ga-rich region    -   221 b barrier body region    -   23 electron blocking layer (p-type layer)    -   24 p-type cladding layer (p-type layer)    -   25 p-type contact layer (p-type layer)    -   26 p-electrode    -   27 n-electrode    -   100 substrate    -   101 AlGaN-based semiconductor layer    -   102 template    -   103 n-type AlGaN-based semiconductor layer    -   104 active layer    -   105 p-type AlGaN-based semiconductor layer    -   106 p-type contact layer    -   107 n-electrode    -   108 p-electrode    -   BL boundary line between first region and second region    -   BA boundary region (inclined region)    -   R1 first region    -   R2 second region    -   T terrace    -   TA terrace region

1. A nitride semiconductor ultraviolet light-emitting elementcomprising: a light-emitting element structure part in which an n-typelayer, an active layer, and a p-type layer made of an AlGaN-basedsemiconductor of wurtzite structure are stacked vertically, wherein thenitride semiconductor ultraviolet light-emitting element has a peakemission wavelength within a range of 300 nm to 327 nm, the n-type layeris composed of an n-type AlGaN-based semiconductor, the active layerdisposed between the n-type layer and the p-type layer has aquantum-well structure having one or more well layers composed of aGaN-based semiconductor, the p-type layer is composed of a p-typeAlGaN-based semiconductor, each semiconductor layer in the n-type layerand the active layer is an epitaxially grown layer having a surface onwhich multi-step terraces parallel to a (0001) plane are formed, then-type layer has a plurality of first Ga-rich regions, the plurality offirst Ga-rich regions being stratiform regions uniformly distributed inthe n-type layer with locally lower AlN mole fraction and includingn-type AlGaN regions in which an AlGaN composition ratio is an integerratio of Al₁Ga₁N₂, each extending direction of the first Ga-rich regionson a first plane perpendicular to an upper surface of the n-type layeris inclined with respect to an intersection line between the uppersurface of the n-type layer and the first plane, and an AlN molefraction of an n-type body region outside the stratiform regions in then-type layer is within a range of 54% to 66%.
 2. The nitridesemiconductor ultraviolet light-emitting element according to claim 1,wherein the peak emission wavelength is set within a range of 300 nm to327 nm with a thickness of a boundary region between adjacent terracesof the multi-step terraces of the well layer being within a range of 2unit cells to 4 unit cells in c-axis direction and with AlN molefractions of two AlGaN-based semiconductor layers vertically adjacent tothe top and bottom of the well layer being within a range of 50% to100%.
 3. The nitride semiconductor ultraviolet light-emitting elementaccording to claim 1, wherein the active layer has a multi-quantum-wellstructure including two or more well layers, and a barrier layercomposed of AlGaN-based semiconductor is present between two of the welllayers.
 4. The nitride semiconductor ultraviolet light-emitting elementaccording to claim 1, wherein the barrier layer is composed of anAlGaN-based semiconductor having an AlN mole fraction within a range of50% to 90%, and boundary region parts between adjacent terraces of themulti-step terraces of the barrier layer located at least on the mostp-type layer side between two of the well layers is a second Ga-richregion with a locally lower AlN mole fraction within the same barrierlayer.
 5. The nitride semiconductor ultraviolet light-emitting elementaccording to claim 4, wherein an AlGaN region in which an AlGaNcomposition ratio is an integer ratio of Al₁Ga₁N₂, Al₂Ga₁N₃, Al₃Ga₁N₄ orAl₅Ga₁N₆ exists in the second Ga-rich region of the barrier layer. 6.The nitride semiconductor ultraviolet light-emitting element accordingto claim 1, comprising an underlying part containing a sapphiresubstrate, wherein the sapphire substrate has a main surface inclined bya predetermined angle with respect to the (0001) plane, thelight-emitting element structure part is formed above the main surface,and each semiconductor layer at least from the main surface of thesapphire substrate to the surface of the active layer is an epitaxiallygrown layer having a surface on which multi-step terraces parallel tothe (0001) plane are formed.
 7. A method for manufacturing a nitridesemiconductor ultraviolet light-emitting element having a peak emissionwavelength within a range of 300 nm to 327 nm and comprising alight-emitting element structure part in which an n-type layer, anactive layer, and a p-type layer made of an AlGaN-based semiconductor ofwurtzite structure are stacked vertically, the method comprising: afirst operation of epitaxially growing the n-type layer of an n-typeAlGaN-based semiconductor on an underlying part including a sapphiresubstrate having a main surface inclined by a predetermined angle withrespect to a (0001) plane, and making multi-step terraces parallel tothe (0001) plane appear on a surface of the n-type layer, a secondoperation of epitaxially growing the active layer of a quantum-wellstructure having one or more well layers composed of a GaN-basedsemiconductor on the n-type layer, and making multi-step terracesparallel to the (0001) plane appear on a surface of the well layer, anda third operation of forming the p-type layer of a p-type AlGaN-basedsemiconductor on the active layer by epitaxial growth, wherein in thefirst operation, a target AlN mole fraction of the n-type layer is setwithin a range of 54% to 66% and a plurality of first Ga-rich regionsare grown so as to extend obliquely upward, the plurality of firstGa-rich regions being stratiform regions uniformly distributed in then-type layer with locally lower AlN mole fraction and including n-typeAlGaN regions in which an AlGaN composition ratio is an integer ratio ofAl₁Ga₁N₂.
 8. The method for manufacturing a nitride semiconductorultraviolet light-emitting element according to claim 7, comprising:setting a thickness of a boundary region between adjacent terraces ofthe multi-step terraces of the well layer within a range of 2 unit cellsto 4 unit cells in c-axis direction and AlN mole fractions of twoAlGaN-based semiconductor layers vertically adjacent to the top andbottom of the well layer within a range of 50% to 100% for the peakemission wavelength to be within a range of 300 nm to 327 nm, in thesecond operation, growing the well layer, and in at least one of thefirst operation, the second operation, and the third operation, growingthe two AlGaN-based semiconductor layers vertically adjacent to the topand bottom of the well layer.
 9. The method for manufacturing a nitridesemiconductor ultraviolet light-emitting element according to claim 7,comprising in the second operation, stacking a well layer composed of aGaN-based semiconductor and a barrier layer composed of an AlGaN-basedsemiconductor alternately by epitaxial growth, and forming the activelayer of the multi-quantum-well structure including two or more welllayers, in which multi-step terraces parallel to the (0001) plane appearon each surface of the barrier layer and the well layer.
 10. The methodfor manufacturing a nitride semiconductor ultraviolet light-emittingelement according to claim 9, comprising: in the second operation,setting a target AlN mole fraction of the barrier layer with a range of55% to 90%, and forming a second Ga-rich region having a locally lowerAlN mole fraction within the same barrier layer in boundary region partsbetween the terraces of the barrier layer located at least on the mostp-type layer side between two of the well layers.
 11. The method formanufacturing a nitride semiconductor ultraviolet light-emitting elementaccording to claim 10, comprising: in the second operation, 1) growingan AlGaN region in which an AlGaN composition ratio is an integer ratioof Al₁Ga₁N₂ in the second Ga-rich region by setting a target AlN molefraction of the barrier layer within a range of 51% to 66%, or 2)growing an AlGaN region in which an AlGaN composition ratio is aninteger ratio of Al₂Ga₁N₃ in the second Ga-rich region by setting atarget AlN mole fraction of the barrier layer within a range of 68% and74%, or 3) growing an AlGaN region in which an AlGaN composition ratiois an integer ratio of Al₃Ga₁N₄ in the second Ga-rich region by settinga target AlN mole fraction of the barrier layer within a range of 76% to82%, or 4) growing an AlGaN region in which an AlGaN composition ratiois an integer ratio of Al₅Ga₁N₆ in the second Ga-rich region by settinga target AlN mole fraction of the barrier layer within a range of 85% to90%.